Sexual interest and activity decrease following castration. We determined by objective criteria the erectile status of 16 men who were sexually active before castration for prostatic cancer. Castration was achieved by orchiectomy or hormonal manipulation. Patients answered a questionnaire regarding the medical status and erectile function before and after castration, and the blood level of testosterone was assessed. During viewing of an erotic videotape penile circumference and erection quality were monitored. Four patients (25%) achieved functional erection. Mean serum free testosterone levels in men who achieved erection were 1.125 +/- 0.362 pg./ml. (standard deviation) and 0.628 +/- 0.098 pg./ml. in those not achieving functional erection (p < 0.001). No statistically significant difference was noted in age, interval since castration, co-morbidity score or method of castration between the men who did and did not achieve erection.