The outcome of 24 months of cabergoline treatment on prolactin (PRL) normalization, tumor shrinkage, restoration of pituitary function, and semen alterations was prospectively investigated in 41 men with macro- (age 17-70 yr) and 10 with microprolactinoma (age 18-53 yr). Fifty-one age-matched men served as controls for semen analysis. At study entry, of the 41 patients with macroprolactinoma, 17 (41.4%) had visual field defects, 14 (34.1%) had headache, eight (19.5%) had galactorrhea, 22 (53.6%) had hypopituitarism apart from hypogonadism, and 30 (73.2%) had low testosterone levels; of the 10 patients with microprolactinoma, none had visual field defects, galactorrhea, or hypopituitarism apart from hypogonadism, two had headache (20%), and five had low testosterone levels (50%; P = 0.3). After 24 months of therapy, 1) PRL levels normalized in 31 patients with macro- (75.6%) and in eight with microprolactinoma (80%; P = 0.9), and galactorrhea disappeared in all patients; 2) maximal tumor diameter reduced by 73.7 +/- 22.6% in macro- and 72.8 +/- 28.3% in microprolactinomas (P = 0.91), and 15 macro- (30%) and seven microprolactinomas (46.7%; P = 0.37) disappeared; 3) visual field defects disappeared in 15 (75%) patients with macroprolactinoma, and headache disappeared in 15 (83%) patients with macro- and in one with microprolactinoma (50%); 4) GH secretion recovered in 62.5% and ACTH secretion in 60% of patients; 5) testosterone levels normalized in 25 patients with macro- (60.9%) and six with microprolactinoma (60%) after 6 months, and 20 patients required testosterone or gonadotropin replacement (in 14 or six patients, respectively); and 6) sperm volume and count normalized in all patients who normalized testosterone levels, whereas motility normalized in more than 80%. Cabergoline therapy was well tolerated; only 4.5% of patients had side effects at high doses. These data demonstrate that cabergoline treatment is as effective and safe in men as in women with prolactinoma and can be successfully used as primary therapy even in men bearing large macroprolactinomas.