Twenty-five men with chronic retention of urine were studied in order to investigate the relationship between upper tract dilatation and bladder pressure. Each underwent assessment of upper tract dilatation and urodynamic investigation before bladder drainage. Patients were reassessed 3 or more months after operation. Residual urine and detrusor contraction pressure decreased significantly after operation (P less than 0.0001:P less than 0.01). A significant increase in effective cystometric capacity and a significant decrease in pressure rise during filling led to an improvement in compliance. At pre-operative assessment, patients with upper tract dilatation had a significantly greater resting bladder pressure (P less than 0.05), end filling pressure (P less than 0.002) and pressure rise during filling (P less than 0.02) than those without upper tract dilatation. In addition, statistically significant correlations were found between serum creatinine and end filling pressure (P less than 0.05) and between upper tract dilatation and both high end filling pressure (greater than 25 cm H2O) and high pressure rise during filling (greater than 15 cm H2O: both P less than 0.05). Whilst high end filling pressure and high pressure rise during filling were found to be closely associated with upper tract dilatation (sensitivity 93 and 100%), there was a high false positive rate (specificity 64 and 45%). Furthermore, statistical analysis demonstrated that these data had a continuous though skewed distribution, suggesting that it is not possible to make a clear distinction between high and low pressure chronic retention.