Factors influencing bladder compliance were examined in 116 patients with benign prostatic hyperplasia (BPH), by evaluating patients' histories, response of isolated bladder strips to acetylcholine, and the effect of prostatic urethral anesthesia. Patients' age, frequency of micturition, and duration of voiding difficulty were not correlated with bladder compliance. Bladder compliance was significantly low in patients within 30 days after urinary retention, as compared with bladder compliance in patients without an episode of retention. More than 30 days after retention, however, there was a tendency toward increased bladder compliance. Restricted to patients without an episode of retention, bladder compliance in the overactive detrusor group was found to be significantly lower than in the normal group. The responses to acetylcholine of bladder strips were compared between patients with low and normalcompliance bladders. The dose-response curve of patients with low-compliance bladders did not differ from that of those with normal compliance bladders, even when patients with an episode of retention were excluded. After prostatic urethral anesthesia, a significant increase of bladder compliance was observed in patients with an overactive detrusor, while the increase was not significant in patients with a normal detrusor. Our results strongly suggest that easy irritability of the anatomically altered prostatic urethra, as well as bladder overdistension caused by urinary retention, are important factors affecting bladder compliance in BPH patients.