A controlled trial has been conducted in which 60 women with mastalgia were randomly allocated to receive tamoxifen at a dosage of either 10 mg or 20 mg daily for either 3 or 6 months. All eligible patients had self-rated moderate or severe mastalgia present for at least 6 months, for which no specific therapy had been given for the previous 3 months. End points of the study were pain control, measured by linear analogue scales, relapse rate and side-effects. Pain relief was achieved in 90 per cent of those receiving 10 mg daily and 86 per cent of those given 20 mg daily. The relapse rate was also similar for both dosages, being 48 per cent and 39 per cent respectively and usually occurred within 2-3 months of discontinuing treatment. However, side-effects were reported less frequently among those receiving 10 mg daily (21 per cent versus 64 per cent; chi 2 = 11.1, P less than 0.001). Prolongation of treatment from 3 months to 6 months did not materially improve the response rate (85 per cent versus 90 per cent). Side-effects were similar, as was the relapse rate among the patients receiving the two durations of treatment. The agent proved to be significantly more effective in the relief of cyclical rather than non-cyclical pain (94 per cent versus 56 per cent). Use of tamoxifen for the treatment of mastalgia is still experimental. Nevertheless, for the majority of women with mastalgia, pain relief can be achieved using tamoxifen 10 mg daily, given for a 3-month course. As almost half these patients will develop relapse of breast pain it may be necessary to give longer courses of therapy, although the safety of this more protracted treatment has yet to be determined.