Sixteen patients aged from 50 to 79 years with inoperable prostatic cancer were allocated at random to treatment either with 300 mg cyproterone acetate at weekly intervals or with 100 mg oestradiol-17 beta-undecylate every month. Blood samples were taken before treatment and at monthly intervals during the 6-month period of treatment. Serum prolactin levels were determined by a double-antibody radioimmunoassay. Basal serum levels of prolactin were 7.76 +/- 4.84 ng/ml and 9.50 +/- 4.32 ng/ml in the two groups of patients before therapy with oestradiol-17 beta-undecylate or cyproterone acetate, respectively. These pretreatment serum prolactin levels were statistically indistinguishable. Serum prolactin levels did not rise significantly during the first 5 months of treatment with cyproterone acetate, although an increase in prolactin was found in the sixth months of therapy with the antiandrogen. On the other hand, oestradiol-17 beta-undecylate raised serum prolactin levels significantly already after one month of treatment and the mean values were significantly higher than in the antiandrogen-treated group. These data combine to suggest cyproterone acetate as a suitable alternative to oestrogens in the therapy of inoperable prostatic cancer.