Chronic fatigue syndrome (CFS), a controversial clinical entity characterized by severe fatigue and constitutional symptoms, has been associated with a variety of psychiatric disorders. To further understand the psychiatric profile of CFS, the authors compared patients with CFS, multiple sclerosis (MS), and major depression by using diagnostic interviews and self-report measures of Axis I disorders and personality disorders. CFS patients differed from patients with major depression, with significantly less depression and fewer personality disorders. Compared with MS patients, CFS patients did not differ with regard to personality disorders. However, they did have significantly more frequent current depression than MS patients, particularly following onset of their illness.