Twenty-eight men with chronic retention of urine were investigated by means of a standard medium fill cystometrogram and by long-term monitoring of bladder pressure whilst the bladder filled naturally. Effective cystometric capacity was greater in each patient during standard cystometry (297 +/- 180 ml) than during long-term monitoring (99 +/- 100 ml; P less than 0.002). The incidence of detrusor instability was also greater during long-term monitoring than during standard cystometry (P less than 0.01). During filling, the detrusor pressure increased significantly both in standard cystometry (27.9 +/- 22.8 cmH2O; P less than 0.002) and in long-term monitoring (4.9 +/- 6.5 cmH2O; P less than 0.002). The detrusor pressure rise during filling was significantly greater during the standard cystometrogram than during long-term monitoring (P less than 0.002). Even when the detrusor pressure rise was corrected to take account of the different cystometric capacities this difference persisted (P less than 0.05). High end filling pressures are common in men with chronic retention investigated by means of a standard cystometrogram but are not observed during long-term monitoring. Such high pressures are probably due to the unphysiological rates of filling and the large volumes of fluid instilled during standard cystometry.