From May of 2000 to May of 2002, the Centers for Disease Control and Prevention (CDC) sponsored a series of three-day workshops to discuss issues related to the current CFS research definition. Each workshop was attended by approximately 20 invited participants that represented an international mix of scientists, clinicians and medical researchers and approximately 10 CDC staff members. During the first workshop, focus groups were formed to address standardization and utilization of instruments used to classify CFS. Each focus group then prepared a summary report. The process that each focus group used included reliance on clinical and scientific knowledge, brainstorming, consensus building and literature reviews. Each focus group report was presented to all workshop participants for further discussion and was modified if necessary. Interval periods between workshops were used for independent review of relevant literature. The papers were circulated via list-serves and resolved as relevant by group consensus either on-line or during the subsequent workshop. Workshop summaries and focus group reports were analyzed and compiled into the recommendations presented here. Where recommendations for specific evaluation instruments were made, wherever possible we favored those that were freely available in the public domain and validated across various language and cultural groups.