The incidence of thromboembolic complications is increased in patients with oestrogen-treated prostatic carcinoma. Because reduced antithrombin-III (AT-III) levels are associated with increased risk of thromboembolism we have determined AT-III concentrations during oestrogen therapy and other treatments. Forty-six patients with carcinoma of the prostate were allocated to either treatment with subcapsular orchiectomy, oestrogen administration, or cyproterone acetate, AT-III was determined before treatment, at 2 weeks and 2 months later. During oestrogen therapy there was a significant reduction in AT-III to 77% of the base-line value. No significant changes were found after orchiectomy. During cyproterone-acetate treatment there was a slight but significant increase in AT-III at 2 months. The reduction in AT-III could indicate an increased risk of thromboembolism during oestrogen treatment of patients with carcinoma of the prostate. On the other hand, the unchanged AT-III levels after orchiectomy and the increased levels during cyproterone acetate therapy could mean that the risk of thromboembolism is less with these two forms of treatment.