We examined the in vivo and in vitro production of prolactin (PRL) in 20 untreated HIV-infected men compared to 14 uninfected men and its association with the cell cycle and apoptosis. Compared to uninfected men, the HIV-infected men had: (i) higher fasting serum bioactive (BIO) PRL; (ii) lower serum immunoreactive (RIA) and BIO-PRL responses to intravenous metoclopramide; (iii) greater BIO-RIA PRL ratio both fasting and during intravenous metoclopramide; (iv) lower percentage of non-stimulated PBMC in the G0/G1 phase, but a higher percentage in the S phase, of the cell cycle with normal response to Concanavalin-A; and (v) higher in vitro production of BIO-PRL by non-stimulated PBMC, which was blocked after Concanavalin-A. Fasting serum BIO-PRL positively correlated with the percent of non-stimulated PBMC in S + G2/M phases. The percentage of apoptotic PBMC negatively correlated with CD4+ T lymphocytes and with the area under the serum RIA-PRL curve, but positively correlated with the area under the curve for the BIO/RIA ratio. These results suggest that in these HIV-infected men: (i) a diminished dopaminergic tone may exist, as an adaptive mechanism attempting to survive; and (ii) BIO-PRL may participate as a cofactor in the stimulation of T-cell proliferation.