The methods used in the early phases of the cohort have been described elsewhere (Barros et al., 1990 , 2001 (link); Victora et al., 1992 ). The study began as a perinatal health survey (Barros, 1986 ) including all 6,011 infants born in three maternity hospitals (accounting for 99.2% of all births in the city). The 5,914 live-born infants were weighed with regularly calibrated pediatric scales (Filizolla, Brazil) to the nearest 10g. Birth length was not recorded. Mothers were weighed and measured and answered a short questionnaire on socioeconomic, demographic, and health-related variables.
The cohort children were followed up at several points in time (Table 1 ). Initially, those born from January to April 1982 were targeted in the 1983 Follow-up Study through the addresses obtained during the hospital interview, when they were aged 8-16 months (mean age 11.3 months).
To calculate the proportion of children located in each follow-up visit, those known to have died were added to those examined. In 1983, 66 of the 1,919 children born from January to April 1982 were know to have died; added to the 1,457 whose mothers or caretakers were interviewed, these accounted for 79.3% of the children who were targeted. Address errors were the main reason for non-response. Non-response rates quoted in the present paper are slightly different from those presented in earlier publications (Barros et al., 1990 ), since they did not fully account for children who had died.
The subsequent phases included the 1984 Follow-up Study (January-April 1984), when the mean age was 19.4 months (range 12-29), and the 1986 Follow-up Study (December 1985-May 1986) (mean age 43.1 months; range 35-53). To minimize losses to follow-up, in each round the approximately 70,000 urban households were visited in search of children born in 1982. After the census was completed, children who still had not been located were searched for at their last known address. This approach resulted in locating 87.2% and 84.1% of the original cohort, respectively.
During each visit, the mother or caretaker answered pre-coded, standardized questionnaires (seeTable 2 for a list of the main variables collected). Children were weighed with portable spring scales (CMS, United Kingdom) and were measured with locally made AHRTAG stadiometers (Barros & Victora, 1998 ). Standard weighing and measuring methods were used (Jelliffe, 1966 ), and the interviewers were extensively trained before fieldwork. Quality-control measures included repeating some 5% of the interviews and measurements by a fieldwork supervisor, standardization sessions, and double data entry.
Two sub-studies were also performed. The Psychomotor Development Study (Victora et al., 1990 ) was performed on a random sub-sample of 360 children born from January to April 1982 who had been seen at all follow-up visits. At the mean age of 4.5 years, they were tested with the Griffiths's scales in six areas of development (locomotor, personal-social, hearing and speech, eye-hand coordination, performance, and practical reasoning).
The Mortality Study entailed the identification of all deaths occurring among cohort children. All hospitals, cemeteries, civil records offices, and the Regional Secretariat of Health were regularly visited from 1982 to 1982 to detect deaths of children and adolescents belonging to the 1982 cohort; from 1987 onwards, it became clear that civil records offices were detecting all deaths, and thus other sources were no longer monitored. Causes of death were investigated by reviewing case notes from outpatient clinics and hospitals, in addition to interviewing family members and the attending physicians. During the interviews a full history of the events preceding the death was obtained with the help of a questionnaire based on that used during the Inter-American Investigation of Mortality in Childhood (Puffer & Serrano, 1973 ). Two independent referees assigned the causes of death using this information; in case of discordance a third senior referee made the final decision. Causes of death were coded according to the Portuguese edition of the International Classification of Diseases, 9th version (OMS, 1980 ). This study is still ongoing.
The cohort children were followed up at several points in time (
To calculate the proportion of children located in each follow-up visit, those known to have died were added to those examined. In 1983, 66 of the 1,919 children born from January to April 1982 were know to have died; added to the 1,457 whose mothers or caretakers were interviewed, these accounted for 79.3% of the children who were targeted. Address errors were the main reason for non-response. Non-response rates quoted in the present paper are slightly different from those presented in earlier publications (Barros et al., 1990 ), since they did not fully account for children who had died.
The subsequent phases included the 1984 Follow-up Study (January-April 1984), when the mean age was 19.4 months (range 12-29), and the 1986 Follow-up Study (December 1985-May 1986) (mean age 43.1 months; range 35-53). To minimize losses to follow-up, in each round the approximately 70,000 urban households were visited in search of children born in 1982. After the census was completed, children who still had not been located were searched for at their last known address. This approach resulted in locating 87.2% and 84.1% of the original cohort, respectively.
During each visit, the mother or caretaker answered pre-coded, standardized questionnaires (see
Two sub-studies were also performed. The Psychomotor Development Study (Victora et al., 1990 ) was performed on a random sub-sample of 360 children born from January to April 1982 who had been seen at all follow-up visits. At the mean age of 4.5 years, they were tested with the Griffiths's scales in six areas of development (locomotor, personal-social, hearing and speech, eye-hand coordination, performance, and practical reasoning).
The Mortality Study entailed the identification of all deaths occurring among cohort children. All hospitals, cemeteries, civil records offices, and the Regional Secretariat of Health were regularly visited from 1982 to 1982 to detect deaths of children and adolescents belonging to the 1982 cohort; from 1987 onwards, it became clear that civil records offices were detecting all deaths, and thus other sources were no longer monitored. Causes of death were investigated by reviewing case notes from outpatient clinics and hospitals, in addition to interviewing family members and the attending physicians. During the interviews a full history of the events preceding the death was obtained with the help of a questionnaire based on that used during the Inter-American Investigation of Mortality in Childhood (Puffer & Serrano, 1973 ). Two independent referees assigned the causes of death using this information; in case of discordance a third senior referee made the final decision. Causes of death were coded according to the Portuguese edition of the International Classification of Diseases, 9th version (OMS, 1980 ). This study is still ongoing.
