From November 1976 to June 1982, a randomised clinical trial was carried out at the Christie Hospital, Manchester, to test the clinical efficacy of tamoxifen (TAM) as an adjuvant to surgery for patients with operable breast carcinoma. Following surgery, premenopausal women were randomly allocated to have either TAM 20 mg day-1 for one year or an irradiation menopause (the previous standard treatment). Postmenopausal women had TAM 20 mg day-1 for one year or no further treatment (Controls). A total of 1005 patients were entered into the trial of whom 961 are evaluable at 10 years from the inception. At 10 years the analysis shows no significant difference in overall and disease free survival between premenopausal women given TAM or an irradiation menopause. For premenopausal node negative patients there would appear to be a trend in favour of the TAM treated patients with a 93% ten year survival vs. 82% for the irradiation menopause group (P = 0.09). When the disease free survival of all 961 patients is analysed, allowing for node status, then there is a marked trend in favour of the TAM treated patients (P = 0.07). Of the patients originally allocated to TAM 47% had an irradiation menopause on relapse and 73% of the postmenopausal control patients had TAM on relapse. The incidence of side effects and second primary tumours is discussed as well as the possible effects of varying the length of time over which adjuvant TAM is administered.