Respondents with a CFS-like illness and a random sample of nonfatigued persons were invited to participate in a clinical examination. They were telephoned to schedule a clinic appointment, with financial compensation ($35) for travel to the clinic. Clinic participants were mailed an informed consent form and a questionnaire asking about the onset of fatigue, the severity and duration of symptoms, current energy level, medical history, and current medications. At the clinic, a trained interviewer confirmed that the subject understood the nature of the study, witnessed the informed consent document, and reviewed the mailed questionnaire for completeness and consistency of responses. The interviewer then privately administered a questionnaire about fatigue16 and modules from the Diagnostic Interview Schedule for Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition17 (modules included somatization, specific phobias, panic disorder, generalized anxiety disorder, major depressive episode and dysthymia, manic episode, schizophrenia, anorexia nervosa, and bulimia nervosa). A physician assistant then performed a standardized physical examination.18 Blood and urine specimens were obtained for laboratory screening tests to identify possible underlying or contributing medical conditions, as stipulated by the CFS case definition.1 Laboratory tests administered to all participants included blood chemistry tests (to determine the levels of glucose, calcium, electrolytes, uric acid, serum urea nitrogen, and creatinine), liver function tests (to determine the levels of aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, albumin, globulin, total protein, lactate dehydrogenase, and total bilirubin), thyroid function tests (to determine the levels of thyrotropin, thyroxine, and triiodothyronine), the determination of the sedimentation rate, an autoimmune screen (for antinuclear antibody and rheumatoid factor), a transferrin saturation test, and a urinalysis (including the determination of urine protein, glucose, and blood levels and a microscopic examination). Participants were offered human immunodeficiency virus testing. Participants or their physicians were notified by mail of any significant abnormalities identified by the laboratory testing or physical examination to ensure appropriate clinical follow-up.