A randomised clinical trial has been conducted to compare adjuvant tamoxifen, 20 mg daily, with tamoxifen and prednisolone, 7.5 mg daily, in post-menopausal women with operable breast cancer. There were 254 evaluable patients, of whom 128 were given tamoxifen alone and 126 received tamoxifen and prednisolone. After a median follow-up of 48 months there was no significant difference in relapse-free or overall survival of the two groups. Furthermore, with survival slightly favouring tamoxifen, confidence intervals on the hazard ratio established that a difference in favour of tamoxifen plus prednisolone of even 5% at 5 years was very unlikely (P < 0.02). Thus, despite the relatively small number of patients in this trial, the data clearly establish that prednisolone is not of value as an additional adjuvant agent.