Thirty men undergoing prostatectomy for symptoms of bladder outflow obstruction and low measured maximum flow rates (20 before and 10 after operation) were studied by means of urodynamic investigation. Paired studies were performed on each patient using a large catheter assembly (4 and 10 F) and a small catheter assembly (epidural line, outside diameter 1.1 mm). The order in which the studies were performed was varied randomly. Detrusor pressure at maximum flow rate was significantly greater in the large catheter study (73 +/- 30 cm H2O) than in the small catheter study (65 +/- 27 cm H2O; P less than 0.003). The maximum flow rate was significantly smaller in the large catheter study (8.9 +/- 9.5 ml/s) than in the small catheter study (12 +/- 7 ml/s; P less than 0.001). The increase in detrusor pressure at maximum flow that was noted during the large catheter study was confirmed in the 20 men who were studied before prostatectomy (mean increase 11 +/- 11 cm H2O; P less than 0.001) but no difference was found between the two methods in the 10 men studied after prostatectomy (50 +/- 19 cm H2O and 49 +/- 15 cm H2O). Using a large catheter assembly to perform urodynamic investigations has the advantage that repeated studies can be performed without recatheterisation, but it has the disadvantage of producing a small increase in detrusor pressure at maximum flow in men with symptoms of bladder outflow obstruction. Few errors in diagnosis should result, however, if laboratories using such catheters are aware of this effect and establish their own limit of normal for voiding pressures.