PHSKC STD clinic providers use a structured form to record client histories before HIV testing. In a face-to-face encounter, clinicians collect information on demographic variables and self-reported substance use, STD history, and sexual behavior. Clinicians ask all clients whether they have had sex with men, women, or both in the prior year. We classified men who reported sex with another man in the prior year as MSM. For MSM, sexual behavior data include the number of male sex partners (anal and oral combined) and how often clients used condoms (always, usually, sometimes, never) during insertive and receptive anal intercourse with HIV-positive, HIV-negative, or HIV-unknown partners. These data use a 12-month recall period. In contrast, for substance use history, clinicians ask MSM whether they have ever used methamphetamine, injection drugs, inhaled nitrites, or crack and, if so, to provide the most recent month and year they used these substances. Syphilis was diagnosed using rapid plasma reagin (RPR) confirmed with Treponema pallidum particle agglutination (TPPA) assay. Urethral gonococcal and chlamydial infections were diagnosed using Aptima Combo 2 (Genprobe, Inc., San Diego, CA) or culture, while rectal and pharyngeal gonococcal and chlamydial infections were diagnosed by culture. Gonococcal cultures used modified Thayer-Martin media and chalmydial cultures used McCoy cells.