Two hundred and fifty-three men undergoing prostatectomy were studied by means of urodynamic investigation to determine the relationship between the volume of residual urine and detrusor function. Increased volumes of residual urine were associated significantly with increasing age, low peak flow rate and high urethral resistance. However, weak voiding pressures were uncommon and did not correlate with increased residual volumes. Increased residual volumes also correlated significantly with a high resting detrusor pressure and a high detrusor pressure rise during the filling phase, but there was a wide scatter of the results for pressure rise and patients could not be classified into groups on the basis of this parameter. Whilst detrusor decompensation was not manifest by weak voiding pressures in patients with increased residual volumes, impairment of the ability of the detrusor to empty the infused cystometric capacity during the subsequent voiding study correlated with increased residual volumes. These data are consistent with theories relating bladder outflow obstruction to the development of increased volumes of residual urine in men undergoing prostatectomy.