Although the aetiology of cyclical mastalgia is poorly understood, the consistent finding of an increased prolactin stimulation response probably due to oestrogen dominance has led to the use of treatment with prolactin-lowering drugs and antioestrogens. The efficacy and safety in cyclical mastalgia of gestrinone, which has androgenic, anti-oestrogenic, and antiprogestagenic properties, were investigated in a multicentre study. In a double-blind randomisation procedure, 72 patients were allocated placebo and 73 treatment with gestrinone (2.5 mg twice a week) for 3 months. The patients recorded the severity of breast pain on a visual analogue scale before and during treatment and scored other breast symptoms as not present (0), mild (1), moderate (2), or severe (3). The gestrinone group had significantly greater reductions than the placebo group in breast pain score at months 1, 2, and 3 of treatment (mean reduction 59.5 [SD 22.6] to 11.7 [17.0] vs 58.2 [17.6] to 36.7 [23.0] at month 3; p less than 0.0001). All six breast symptoms had lower scores in the gestrinone than in the placebo group by the end of treatment. In a subset of 30 participants (15 from each group), serum concentrations of oestradiol, progesterone, and tri-iodothyronine were significantly lower than baseline after 3 months of gestrinone, but concentrations of luteinising hormone, follicle-stimulating hormone, prolactin, thyroid-stimulating hormone, and thyroxine did not change. 41% of gestrinone-treated and 14% of placebo-treated patients reported at least one side-effect; most of these were androgen-mediated. 11 placebo-treated patients and 4 on gestrinone discontinued treatment. Thus, gestrinone was very effective in the treatment of cyclical mastalgia and was well tolerated.