Prolactin is an important physiological regulator of prostate development and growth in preclinical models. In prostate cancer there is strong evidence that prolactin exerts a trophic effect independent of testosterone. In addition, patients with prostate cancer that have an elevated prolactin level correlated with a poorer prognosis. Based on these data, we evaluated the clinical effect of prolactin suppression using bromocriptine in patients with androgen-independent prostate cancer. We conducted an open-label phase II trial of bromocriptine in patients with progressive metastatic prostate cancer. Basal and thyrotropin releasing hormone (TRH)-stimulated prolactin levels were utilized as biological endpoints for determining the dose of bromocriptine. All patients continued to receive complete androgen blockade. Thirteen patients were enrolled (median age 69.5 years). There were no complete or partial responses associated with bromocriptine in 11 of the evaluable patients. The mean duration of bromocriptine treatment was 8.2 weeks (2-14 weeks). One patient had a clinically insignificant decrease in prostate-specific antigen (PSA) and another patient had a 19.9% decrease in PSA with progression of a soft tissue mass. The vast majority of patients (10 of 11) had suppression of prolactin with a bromocriptine dose of 2.5 mg three times a day. One patient required a dose adjustment due to inadequate suppression, with a final maintenance dose of bromocriptine 12.5 mg per day resulting in complete suppression. No serious treatment-related toxicities were observed. The most common complications noted were nausea, headaches, dizziness, and fatigue. Our data showed that 2.5 mg three times per day of bromocriptine suppressed prolactin in 90% of the patients. Furthermore, this dose appears to be well tolerated.