The clinical and laboratory findings from studies of patients with chronic fatigue syndrome (CFS) from northern Nevada are summarized. Physicians caring for these patients have estimated that >400 patients with CFS from northern Nevada and nearby communities in California were identified between 1984 and 1988.As a result of these studies, a cluster of clinical and laboratory features associated with the illness in moderately to severely affected patients has been identified: profound fatigue of prolonged duration; cervical lymphadenopathy; recurrent sore throat and/or symptoms of influenza; loss of cognitive function manifested by loss of memory and loss of ability to concentrate; myalgia; impairment of fine motor skills; abnormal findings on magnetic resonance imaging brain scan; depressed level of antibody to Epstein-Barr virus (EBV) nuclear antigen; elevated level of antibody to EBV early antigen restricted component; elevated ratio of CD4 helper to CD8 suppressor cells; and strong evidence of association of this syndrome with infection with human herpesvirus 6. More-serious and longer-lasting neurologic impairments, including seizures, psychosis, and dementia, have also been observed in some of these patients.