In a randomised parallel-block trial in thirteen European centres bromocriptine 2.5 mg twice a day was compared with placebo therapy for cyclical mastalgia. 272 patients were enrolled into the study. Linear analogue charts and diary pain cards were used for assessment of response. Reduction in breast pain, heaviness, tenderness, and serum prolactin after 3 and 6 months' therapy were significantly greater with bromocriptine than with placebo. Improvement in symptoms with bromocriptine was maintained for at least 6 months after therapy. Overall 29% of patients dropped out while on therapy, more from the bromocriptine than from the placebo group. Adverse effects, especially nausea and dizziness, were commoner among the bromocriptine-treated patients, but blood pressure was unaffected.