Clinical and laboratory evaluations of 181 men were performed; the population studied included 131 homosexual or bisexual men (who were stratified into one of three groups on the basis of symptomatology and abnormal findings on physical examination), 39 heterosexual men (who served as controls), and 11 men with acquired immunodeficiency syndrome (AIDS). The presence of specific symptoms and/or lymphadenopathy in homosexual men was associated with perturbation of immunoregulation. A large percentage of asymptomatic, homosexually active men had abnormalities in lymphocyte numbers, subpopulations, and subclasses. Symptomatic individuals had clinical findings, immunologic alterations, and abnormalities of hematopoiesis similar to those found in patients with AIDS. These data suggest that clinical evaluation in conjunction with laboratory studies may serve to identify individuals at increased risk of developing AIDS.