Strain voiding has been reported to be a frequent symptom following radical prostatectomy. However, pathophysiology of vesicourethral function underlying voiding difficulty has not been well studied. In the present study, we investigated detrusor underactivity following radical prostatectomy. The records on urodynamic study (pressure-flow study, urethral pressure profile) were retrospectively investigated in 80 patients undergoing laparoscopic radical prostatectomy and all urodynamic studies pre- and post-operatively. We extracted the cases with detrusor underactivity according to the criteria of overt strain voiding pattern on post-operative pressure flow study; detrusor pressure at the maximum flow rate (Pdet Q(max)) of less than 10 cmH2O in conjunction with an increase of abdominal pressure. Of the 80 patients, 6 (7.5%) were found to have detrusor underactivity. In all patients, good detrusor contraction was confirmed on the pre-operative urodynamic study performed before surgery. On the voiding phase of pressure-flow study in these patients, mean Pdet Q(max) showed a significant decrease postoperatively from 58.5 cmH2O to 3.0 cmH2O (p < 0.01), although mean abdominal pressure at Q(max) significantly increased from 24.2 cmH2O to 105.8 cmH2O (p < 0.05). Mean Q(max) on free uroflowmetry showed a significant increase from 12.8 ml/sec to 22.1 ml/sec (p < 0.05). No patient had significant post-void residual urine. On the storage phase of the study, however, maximum cystometric capacity, maximum urethral closing pressure showed no significant change between pre- and post-operative studies. Five patients acquired continence and one had mild urinary incontinence using one pad a day. The present study showed that detrusor contaractility could be impaired during radical prostatectomy, but, no apparent operative procedure related to detrusor dysfunction could be identified in the present patients.