In experimental models, prolactin (PRL) displays independent hypertrophic effects on the prostate. To investigate whether hyperprolactinemia is associated with prostate enlargement in humans, we designed this open, prospective, case-control study enrolling 20 men with prolactinoma (aged 34 +/- 10 yr) and 20 age-matched healthy men. The endocrine profile and prostate transrectal ultrasonography were performed before and after 12 and 24 months of cabergoline treatment in the patients and at study entry and after 24 months in the controls. The patients had lower serum testosterone, dihydrotestosterone (DHT), and IGF-I levels and prostate volume (15.4 +/- 3.5 vs. 19.6 +/- 5.1 ml; P < 0.001) and higher PRL levels and prostate-specific antigen density than controls. There was no difference in prostate and transitional zone volumes between patients with normoandrogenemia (n = 8) or hypoandrogenemia (n = 12). After 12 and 24 months of treatment, PRL, testosterone, and DHT levels were normal in all cases, as were IGF-I and IGF-binding protein-3 levels. After 24 months, prostate volume was comparable to that in controls (21.7 +/- 4.5 vs. 22.5 +/- 4.7 ml). There were no changes in prostate structure throughout the study period in either the patients or the controls. In conclusion, in young men with prolactinoma PRL excess is unlikely to have effects on the prostate per se, because it is accompanied by low testosterone and DHT levels that produce the major effects.