Prolactin (PRL) is an immunostimulatory hormone that is increased in serum of patients with systemic lupus erythematosus (SLE). On the other hand, SLE and AIDS patients share some clinical and immunologic manifestations and probably share similar pathogenic factors and etiology. Based on the above, we decided to investigate the serum PRL levels in 22 asymptomatic HIV-infected patients, 32 patients with AIDS, 9 seronegative homosexual men, and 66 healthy controls. Antibodies to HIV were determined by a micro-ELISA system and by HIV-1 Western blot. Serum PRL was assayed by monoclonal antibodies. Serum PRL levels in the control group were 5.07 +/- 1.36 ng/ml and in the seronegative homosexual men were 5.81 +/- 1.45 ng/ml. Serum PRL levels in asymptomatic HIV-infected patients were 9.22 +/- 3.3 ng/ml and in AIDS patients were 10.26 +/- 2.5 ng/ml (P < 0.01 and P < 0.001, respectively, for both groups when compared to healthy controls). The difference between seronegative homosexual men and AIDS patients was P < 0.05. In the present study we found that serum PRL levels in AIDS patients are higher than in seronegative homosexual men and healthy controls; these results are in agreement with a previous published report. Since PRL is related to lymphocyte activation and lymphoproliferation, the finding may have monitoring, prognostic, and therapeutic implications.