The effects of surgery on the immune response (as measured by a variety of parameters) were examined in patients undergoing transurethral resection of the prostate (TURP) for benign disease. Two groups were assessed, one having the procedure performed under a general anaesthetic and the other under a spinal anaesthetic. A significant reduction in lymphocyte numbers and in the response of lymphocytes to the mitogens PPD and PWM and to histocompatibility antigens in mixed lymphocyte culture was seen in patients after TURP under a general anaesthetic but minimal changes were found in the patients who had a spinal anaesthetic. Cortisol levels were not elevated in either group after surgery. The immunodepression occurring after TURP appears to be due to general anaesthesia rather than the trauma of the surgery.