We investigated 41 men with chronic retention of urine owing to bladder outflow obstruction by long-term monitoring of bladder pressure and conventional cystometry to determine the relationship between detrusor pressure and upper tract dilatation. We confirmed that high pressures during conventional filling cystometry were common in men with upper tract dilatation. However, important differences were demonstrated between long-term monitoring and conventional cystometry. The pressure increase during the natural filling phase of long-term monitoring was significantly smaller than that during conventional cystometry. Detrusor instability was found in 88 per cent of the men during long-term monitoring but in only 51 per cent during conventional cystometry (p less than 0.001). High frequency unstable detrusor contractions during long-term bladder pressure monitoring were associated significantly with upper tract dilatation (p less than 0.0001) and correlated significantly with impairment of glomerular filtration rate (rs equals -0.7339, p less than 0.001).