The aim of this study was to investigate urodynamic findings and the outcome of surgical incision in patients with nighttime wetting due to urethral obstruction. A total of 239 patients with nighttime wetting (157 males and 82 females; mean age, 8.7; range, 4-18 years) was studied. One hundred and ten patients had monosymptomatic enuresis alone and 129 both nighttime wetting and daytime symptoms, that is, the majority was not simple bedwetters, but had signs of bladder overactivity. All patients were first treated with bladder training and fluid intake after dinner was restricted. The non-responders were then treated with tricyclic antidepressants, anticholinergics, or intra-nasal desmopressin and conditioning therapy. Ninety-seven patients who did not respond to these conventional treatments were investigated by voiding cystourethrography, bougie ร boule, and urodynamic study. Urethral obstruction was noted in 34 patients (14.2%); posterior urethral valves in three males, ring stricture of the bulbar urethra in 26 males and distal urethral stenosis in five females. Detrusor instability was noted in 30 (94%) of 32 patients. Three patients had <15 mL/s of maximum flow rate and none had post-void residual urine. Pressure at maximum flow (PQ(max)) was 106.5 +/- 26.3 cm H(2)O in pressure/flow study. After surgery, nighttime wetting was cured in 25 patients (73.5%), improved in four (11.8%). Detrusor instability disappeared in six of 16 patients studied (37.5%) and improved in 8 (50%), and PQ(max) decreased significantly after operation (P = 0. 0034). In conclusion, most of the patients with urethral obstruction have detrusor instability and high pressure voiding, and these conditions improved after operation.