Prostate cancer is common in aged men and radical prostatectomy is established as a therapeutic measure. However, to date there is little information about its impact on voiding function. We conducted a prospective clinical study to elucidate the impact of radical prostatectomy on voiding function in 17 patients with prostate cancer, by urological evaluation including filling and voiding cystometry (pressure flow study). The patients who were estimated as having weak detrusor function including very weak detrusor function at 3 months postoperatively had significantly more frequent urinary incontinence compared with the others (p < 0.05). Of 8 patients who showed urinary incontinence for more than 3 months, 7 (88%) patients developed weak detrusor function at 3 months after operation, but 4 of them were estimated as having normal detrusor function preoperatively. These patients revealed reduced maximum flow rate and significantly increased quality of life score compared with the other patients (p < 0.05). An initially reduced bladder compliance disclosed a tendency to a rapid return to normal with time after surgery. Detrusor overactivity itself and neoadjuvant antiandrogen therapy were not related to prolonged postoperative urinary incontinence. The present study indicates that caution is required when administering medication that could potentially affect detrusor function, regardless of the type of preoperative detrusor function, in patients with persistent urinary incontinence or a reduced urinary stream. Particular emphasis is laid on the importance of urodynamic assessment of post-prostatectomy detrusor function and appropriate management modalities based on the results.