This study describes the urodynamic findings in 22 patients with posterior urethral valves and discusses their association with urinary incontinence, age, mode of primary treatment, renal function, and changes in the upper tracts. The patients' ages ranged from 3 to 26 years and 27% were either adolescents or older. The urodynamic findings were categorized into 5 main patterns, although mixed patterns were also observed; (1) normal capacity and compliance with normal detrusor contractility (2/22 patients, 9.1%); (2) small-capacity, hypocompliant bladder (8/22 patients, 36.4%); (3) unstable bladder (2/22 patients, 9.1%); (4) large-capacity, hypotonic bladder with decreased detrusor contractility (2/22 patients, 9.1%); and (5) normal capacity and compliance but with decreased detrusor contractility (8/22 patients, 36.4%). More than one-half of the patients (57.1%) evacuated their bladders incompletely, and this seemed to be associated with post-treatment episodes of urinary-tract infection. The commonest symptom was daytime frequency, urgency, and leak with nocturnal enuresis, which urodynamically correlated with a small-capacity, hypocompliant or unstable bladder or to incomplete evacuation of the bladder, leading to significant post-void residue. Significant detrusor dysfunction was identified in 2 asymptomatic patients as well, emphasizing the need to perform a routine urodynamic work-up on all valve patients. Urodynamic properties seemed to be associated with age. Small, hypocompliant, and unstable bladders were almost always seen in prepubertal boys and in the first 5 years following undiversion, whereas large, hypotonic bladders with impaired contractility were seen in post-pubertal boys. While the current policy is to avoid high diversion, data in this study suggest that disorders of detrusor capacity, compliance, and contractility exist in children treated by primary valve ablation and vesicostomy and that abnormal detrusor dynamics seem to be a reflection of inherent developmental detrusor dysfunction consequent to congenital infravesical obstruction.