The effects of oestrogen therapy and of orchidectomy on coronary status, as reflected by exercise ECG-testing before and after one year of treatment, were assessed in a randomized study of patients (N = 100) with prostatic cancer. Oestrogen was given as polyestradiol phosphate 80 mg i.m. per month in combination with 150 micrograms ethinylestradiol p.o. per day. There were no significant inter-group differences in conventional risk factors or in pre-treatment exercise test results. Twelve months after the start of therapy the oestrogen group showed a significantly greater depression of the ST-segment during maximal exercise in leads CH2 (P less than 0.0005) and CH5 (P less than 0.01) compared with the pre-treatment depression. Twenty-five per cent (N = 13) of the patients in the oestrogen group suffered cardiovascular complications during the year of therapy, whereas no such complications were observed in the orchidectomy group. However, even the patients in the oestrogen group who had not suffered cardiovascular complications had significantly greater depressions of the ST-segment during exercise both in lead CH2 (P less than 0.0005) and in CH5 (P less than 0.05). There was no significant change in the ST-segment level in the orchidectomy group twelve months after surgery. In summary, we found evidence of an induction of myocardial ischaemia during treatment with exogenous oestrogens at low dosage in patients with prostatic cancer. This deleterious effect of oestrogen on the coronary status argues against oestrogen therapy, since oestrogen has not been shown to be more beneficial than orchidectomy against prostatic carcinoma.