Chronic fatigue syndrome (CFS), an illness characterized by debilitating fatigue and a number of associated symptoms, was identified in 135 patients using the case definition provided in 1988. The demographic features of these patients, 97% of whom resided in Minnesota, were similar to those reported elsewhere. About three-fourths of the cases occurred between 1984 and 1989, and in 123 (91.1%), the illness began with what appeared to be an acute infection. Patients had been ill for an average of 4.3 years before enrollment in the study. Fatigue was their most troublesome symptom, although a majority of the patients rated most of the general symptoms and neuropsychological complaints associated with CFS as moderate or severe. Follow-up data obtained on 62 patients one year after initial evaluation revealed that none had completely recovered. However, about 40% reported some improvement in each of the CFS symptoms.