In 1984 a large prospective study of gay and bisexual men was begun to elucidate the natural history of the human immunodeficiency virus (HIV) infection. At two successive semiannual examinations, clinical or hematologic abnormalities were found up to 13 times more often among HIV-seropositive men (n = 1611) than HIV-seronegative men (n = 2646). More than 30% of the seropositive participants had persistent generalized lymphadenopathy, independent of T-helper lymphocyte (CD4) counts and most other signs and symptoms. Other clinical manifestations such as thrush, anemia, thrombocytopenia, neutropenia, fever, and fatigue occurred with only slightly reduced CD4 counts (400 to 700/mm3) and appeared to increase exponentially with progressively lower counts. A simple systematically derived clinical index using these manifestations identified more than 70% of the seropositive men with significant T-helper cell depletion. This kind of clinical index may be useful for assessing groups of HIV-infected persons, especially those whose T-lymphocyte numbers and function cannot be readily measured.