Herpesvirus infections were studied in persons with or at risk for the acquired immune deficiency syndrome (AIDS). The infections diagnosed were as follows: patients with AIDS, cytomegalovirus (CMV) in 34 of 34, Epstein-Barr virus (EBV) in 33 of 34, herpes simplex viruses (HSV) in eight of 34, and varicella zoster virus in four of 34; patients with chronic lymphadenopathy syndrome, CMV in eight of nine and EBV in nine of nine; in healthy homosexual men, CMV in five of 13 and EBV in seven of eight. Cytomegalovirus infections were frequently related to disease and death. Herpesvirus infections are frequent causes of serious diseases in AIDS. The prevalence of EBV and CMV infections in AIDS and the chronic lymphadenopathy syndrome may be the result of an important interaction between these viruses and the cause of AIDS.