Preoperative water cystometrograms in 437 patients with benign prostatic hyperplasia (BPH) were examined in a retrospective study. The cystometrographic results were analyzed regarding the preoperative clinical features: patient age, presence or absence of urinary incontinence, history of urinary retention and rate of residual urine. The prognostic value in improvement in voiding difficulty and postoperative urinary incontinence was also analyzed at 1 and 6 months after elective prostatectomy. Subjective symptoms of the patients were the primary reasons for prostatectomy, the majority of which were performed by a single competent resectionist (K. T.) who evaluated the outcome but was blinded to the cystometric findings. Of these patients 263 (60.2%) had detrusor instability (group 1), while 174 did not (group 2). Vesical denervation supersensitivity to bethanechol chloride was noted in 47 of 375 patients (12.5%). The difference in clinical features was significant between the 2 groups, with group 1 showing older patient age (p < 0.01), and a greater incidence of urinary incontinence (p < 0.001) and retention (p < 0.001). The difference between groups 1 and 2 in mean bladder capacity (p < 0.01), compliance (p < 0.01) and a greater positive rate of vesical denervation supersensitivity (p < 0.001) was also significant. The clinical and cystometrographic parameters studied worsened with advancing patient age. Although the majority of the patients (94.7%) were relieved of obstructive symptoms after transurethral prostatectomy (6 months later), 113 (25.9%) were not at 1 month. Compared to 324 patients with early improvement (74.1%), those without improvement at 1 month were characterized by older age (p < 0.01), greater prevalence of preoperative incontinence (p < 0.05), retention (p < 0.01), greater residual rate (p < 0.05), a less compliant bladder (p < 0.01) and a higher positive rate of vesical denervation supersensitivity (p < 0.05). Cystometrographic findings, however, had no relevance to late (6 months) outcome of voiding difficulty. On the other hand, postoperative incontinence was noted in 100 patients (22.9%) at 1 month after transurethral prostatectomy, with the majority having episodes similar to those experienced preoperatively (70.0%) as well as detrusor instability (87.0%). They also were older (p < 0.01), and had a less compliant bladder (p < 0.01) and a higher positive rate of vesical denervation supersensitivity (p < 0.01) than did continent patients. Only 18 elderly patients (4.1%) remained incontinent 6 months later, all with a less compliant (p < 0.01) and more unstable (p < 0.01) bladder initially. The genesis of this detrusor dysfunction was believed to be aging in male patients, in whom BPH evolves and progresses. In conclusion, preoperative cystometrography in patients with BPH is valuable in that it correlated well with the clinical features and it can predict to some extent the outcome of obstructive symptoms and urinary incontinence after transurethral prostatectomy.