Forty-five women with mastalgia severe enough to warrant investigation and treatment were recruited to a randomized clinical trial of daily pain diary keeping and relaxation therapy (RT) versus diary keeping on its own (control). All patients kept a pain diary during the study which lasted 12 weeks; women assigned to RT practised daily with an audio-cassette recording during weeks 5-8. Women who received RT improved significantly more than control patients as assessed by the Cardiff Breast Scale (P = 0.05); of those patients staying in the trial for at least 8 weeks, 61% of the women receiving RT, compared with 25% of the control patients, showed a complete or substantial response. Moreover, for women receiving RT, the mean number of pain-free days rose from 9.6 in the first (baseline) month to 14.2 in the third (post-treatment) month (P < 0.05) (cf a nonsignificant increase from 9.2 to 10.5 days in the control patients). Because patients were asked to rate pain in the previous month at all visits, including the first, it was possible to carry out an 'intention to treat' analysis for this variable for all 45 patients: patients allocated to RT reported significantly less pain at week 12 than at trial entry (P = 0.009) whereas this was not the case for women in the control group. Although the numbers are small, patients with cyclical mastalgia who received RT showed a significantly greater improvement on the Cardiff Breast Scale at the end of the trial than those with cyclical mastalgia in the control group (P = 0.05). Furthermore, patients with cyclical mastalgia who received RT rated their pain as less severe during weeks 5-8 (when they were practising relaxation daily) than did the patients with cyclical mastalgia in the control group (P < 0.04). This suggests that women with cyclical pain may show a greater response to RT. Moreover, women receiving RT, but not control patients, were significantly less anxious after treatment. The results of this pilot study suggest that RT may be useful in the management of mastalgia.