In order to understand the influence of obstruction duration on the recovery of obstructed bladders, a functional study of 36 obstructed rabbit bladders after relief of outlet obstruction was done. Bladder outlets were obstructed by placing a 5-mm inner diameter polyethylene cuff around the urethra. Obstructions were kept in place for varying periods: 1 day, 3 days, 1 week, 2 weeks, 4 weeks, 6 weeks and 8 weeks. The obstructions were then released and functional studies were performed after a recovery period of 2 weeks (experimental groups). Using results of another study of obstructed bladders as a basis, the bladder weight of the experimental groups decreased from an initial increase after obstruction, but stabilized at a 2.5-fold increase 8 weeks after release compared with the control bladders. In vitro whole bladder responses to electrical and carbachol stimulations were also studied. Significant functional recovery was noted in the experimental bladders of less than 4 weeks' obstruction, whereas in those of 6 weeks' and 8 weeks' obstruction, no definite functional improvement was noted after relief of outlet obstruction. The expulsion ability was not significantly better in the experimental bladders than in the obstructed bladders. A slight decrease in expulsion ability was noted in the experimental bladders as the obstructive duration increased but the difference was not statistically significant. An obstructive duration of up to 8 weeks did not have a significant influence on the functional recovery of the rabbit bladders. Applying these results to the management of humans with benign prostatic hyperplasia, surgical intervention seems indicated only in those patients with symptomatic outlet obstruction. Stable, mild obstruction can probably be treated conservatively.