In the course of a prospective study of the prevalence and incidence of infection with the human immunodeficiency virus (HIV) and risk factors for the acquired immune deficiency syndrome among 961 homosexual men, 97 initially HIV antibody seronegative men reported a febrile period lasting at least three days. In 60 of these men serological evidence for an infection was found: influenza A or B virus (17 men), HIV (14), Epstein-Barr virus (seven), parainfluenza virus type I, 11 or Ill (five), hepatitis A virus (three), cytomegalovirus (three), adenovirus (two), respiratory syncytial virus (two), hepatitis B virus (one) and Toxoplasma gondii (one). Combined infections were found in five men. A total of 17 men seroconverted for HIV antibody. The clinical symptoms of acute HIV infection closely resembled those of influenza A or B infection. Skin rashes also occurred frequently in men with HIV infection. HIV antibody seroconversion gives rise to a number of different symptoms and primary HIV infection should be included in the differential diagnosis of prolonged febrile illness in those at risk of HIV infection.