To select patients at increased risk of needing prolonged postoperative bladder drainage after incontinence surgery, the data obtained from preoperative uroflowmetry and voiding urethrocystometry were analyzed in a group of 43 patients for presence and/or absence of reduced flow rates and detrusor contraction during voiding. All patients with adequate detrusor contraction and flow rates were able to resume spontaneous voiding by the seventh postoperative day. One third of patients voiding without detrusor contraction needed prolonged bladder drainage (p less than 0.05). Presence of normal flow rates reduced their risk to 20 per cent while reduced flow rates enhanced their risk to 100 per cent (p less than 0.05). Changes observed in urethral pressure, abdominal pressures, and flow rates individually were not significant predictors of the need for prolonged bladder drainage.