A cluster of 134 patients who had undergone Epstein-Barr virus (EBV) serological testing because of suspected chronic EBV syndrome was investigated in Nevada. Fifteen case-patients were identified who had severe, persistent fatigue of undetermined etiology for more than two months. When compared with the remaining 119 patients who had less severe illnesses and with 30 age-, sex-, and race-matched control-persons, these 15 patients had significantly higher antibody titers against various components of EBV and against cytomegalovirus and herpes simplex and measles viruses. Epstein-Barr virus serology could not reliably differentiate individual case-patients from the others, and the reproducibility of the tests within and among laboratories was poor. As a group, the case-patients appear to have had a syndrome that is characterized by chronic fatigue, fever, sore throat, and lymphadenopathy. The relationship of this fatigue syndrome to EBV is unclear; further studies are needed to determine its etiology.