Cabergoline (CAB) is a new oral dopaminergic compound showing a very long-lasting PRL-lowering activity and reported to be well tolerated. The efficacy and tolerability of chronic treatment with CAB in 30 female hyperprolactinemic patients, aged 18-52 yr (6 microadenomas, 3 macroadenomas, and 21 functional hyperprolactinemias), were studied. In a group of 10 patients who received CAB (0.8 mg once weekly or 0.4 mg twice weekly) for 8 weeks PRL levels normalized while on treatment and remained normal (8 patients) or greatly reduced (1 patient) for 1-2 months after discontinuation of the drug. Twenty-six patients underwent chronic treatment (6-12 months) with an initial dose of 0.5 mg once weekly, subsequently increased to 1-2 mg in 10 patients and decreased in the other 2. Due to severe side-effects CAB was discontinued in 3 patients, in 1, 8, and 12 weeks. A significant reduction of PRL levels was already observed after the first week of treatment (mean +/- SEM basal values, 90.1 +/- 13.3 vs. 29.5 +/- 6.3 micrograms/L; P less than 0.001). Twenty-two patients had normal PRL levels in 1-36 weeks (mean, 6 weeks) with 0.5-2 mg CAB. Twenty-two patients resumed regular menses; 2 patients became pregnant after 3-11 months of treatment. Thirteen patients complained of side-effects (nausea, hypotension, headache, gastric pain, dizziness, and weakness) that disappeared with time in 10 of them. The comparison with a previous bromocriptine treatment regimen in 20 patients had shown that the number of patients requiring discontinuation of the latter drug was significantly higher (7 vs. 3 patients; P less than 0.001). However, 2 patients who needed to discontinue CAB were able to tolerate bromocriptine therapy. A computed tomographic scan performed after 12 months of therapy in 7 patients showed a significant reduction (50%) of the adenoma in 5. In conclusion, our results show that CAB is a well tolerated new dopamine agonist with long-lasting activity that represents an advance in chronic medical treatment of hyperprolactinemic conditions.