In an ongoing prospective study of homosexual men conducted in Vancouver since November 1982, 87 cases of human immunodeficiency virus (HIV) seroconversion have been documented to date. Comparison of laboratory results obtained a mean of 4.9 months before and 5.4 months after the estimated date of seroconversion revealed that a significant increase in the serum IgG level (from 1149 to 1335 mg/dl on average) and in C1q binding (from 8.8% to 14.2% on average) was associated with early HIV infection (p less than 0.001). A marginally significant decrease in the ratio of helper to suppressor (CD4 to CD8) cells (from 1.55 to 1.29 on average) was also noted (p = 0.025). A marked decrease in absolute number of CD4 cells was not seen with seroconversion, which suggests that profound loss of these cells may be a long-term effect of HIV infection. The occurrence of symptoms (including fatigue, fever, night sweats, unintentional weight loss, diarrhea, joint pains, cough unrelated to smoking, shortness of breath, oral thrush, herpes zoster and rash) did not increase with seroconversion. This finding suggests that most cases of HIV seroconversion may be asymptomatic or associated with relatively minor symptoms. On the other hand, generalized lymphadenopathy was found to develop after HIV seroconversion in about 50% of cases.