Serum testosterone, 17 beta-oestradiol, luteinising hormone (LH) and prolactin were measured in 28 postmenopausal breast cancer patients after mastectomy and in 30 postmenopausal normal controls. In the patient group, mean levels of oestradiol, LH and prolactin did not differ significantly from those of the control group. Mean testosterone levels were higher in breast cancer patients than in normal controls, either considering the overall groups (P less than 0.001) or dividing them into subgroups according to years since menopause. Breast cancer patients were divided into 2 subgroups according to time since mastectomy: 19 patients had been examined within a year of mastectomy and 9 patients some years after mastectomy. Testosterone, but not oestradiol, LH or prolactin values in each subgroup were still significantly higher (P less than 0.001 and P less than 0.02, respectively) than in normal controls. Years since menopause were significantly correlated with testosterone (r = 0.533, P less than 0.01) but not with the other hormones in the cancer group. These results confirm our previous findings of increased urinary testosterone values in postmenopausal breast cancer patients and support the hypothesis that androgens may play a role in the development of breast cancer.