A survey of preoperative and postoperative urine flow rates, urodynamic data symptoms in patients undergoing transurethral prostatectomy for benign prostatic hyperplasia indicates that there are certain findings that can help predict early and late postoperative symptoms. Long-term bladder outlet obstruction often is associated with detrusor hyperreflexia. In our series, although all patients had relief of the obstruction when postoperative urine flow rates were measured (p less than 0.005), 28 per cent of the patients who had a hyperreflexic detrusor response had an exacerbation of the symptoms of obstruction 4 weeks postoperatively, while none of the patients with a normal detrusor response noted any increase in symptoms. At 3 months postoperatively all patients still complaining of symptoms demonstrated detrusor hyperreflexia. Patients with detrusor hyperreflexia preoperatively tended to revert to a normal detrusor response after the relief of bladder obstruction. It is concluded that the preoperative finding of detrusor hyperreflexia should alert the surgeon to the possibility of prolonged postoperative symptoms, although the detrusor response eventually will become normal and symptoms will disappear in almost all of these patients.