Eighty patients (54-75 years) with cytologically and/or histologically confirmed cancer of the prostate (CAP) were randomly allocated to either orchidectomy (ORX, n = 41) or combined intramuscular and oral estrogen treatment (ESTR, n = 39). Serum levels of prolactin (PRL) were determined prior to treatment and 6, 12 and 24 months after initiation of the treatment. In the ORX group, 32 patients responded to the treatment and 9 did not. In the ESTR group, 27 patients responded and 12 did not. Serum PRL levels were significantly increased by ESTR treatment in responders as well as in nonresponders, but were not affected by ORX. There were no differences in PRL values between responders and nonresponders at any time in any of the two treatment groups. It is concluded that serum PRL assay have no prognostic value in the employed endocrine treatment of CAP.