To determine the medical and psychiatric diagnoses that have an aetiological role in chronic fatigue we conducted a prospective study of 405 (65% women) patients who presented for evaluation with this chief complaint to an academic medical centre. The average age was 38.1 years and the average duration of fatigue at entry in the study was 6.9 years. All patients were given comprehensive physical and laboratory evaluations and were administered a highly structured psychiatric interview. Psychiatric diagnoses explaining the chronic fatigue were identified in 74% of patients and physical disorders were diagnosed in 7% of patients. The most common psychiatric conditions in this series were major depression, diagnosed in 58% of patients, panic disorder, diagnosed in 14% of patients, and somatization disorder, diagnosed in 10% of patients. Primary sleep disorders, diagnosed in 2% patients, and chronic infections, confirmed in 1.6% patients, explained the majority of cases whose chronic fatigue was attributed to a physical disorder. Thirty per cent of patients met the criteria used to define the chronic fatigue syndrome (CFS). Compared with age- and gender-matched control subjects with chronic fatigue, CFS patients had a similarly high prevalence of current psychiatric disorders (78% versus 82%), but were significantly more likely to have somatization disorder (28% versus 5%) and to attribute their illness to a viral infection (70% versus 33%). We conclude that most patients with a chief complaint of chronic fatigue, including those exhibiting the features of CFS, suffer from standard mood, anxiety and/or somatoform disorders. Careful research is still needed to determine whether CFS is a distinct entity or a variant of these psychiatric illness.