OBJECTIVE To assess urodynamic and clinical data in patients with primary enuresis for potential prognostic indicators of detrusor instability. METHODS The records of 33 patients (mean age 8.8 years, range 5-14) with monosymptomatic primary enuresis (MPE, bedwetting as the sole symptom) and 47 patients (mean age 7.1 years, range 5-12) with complicated primary enuresis (CPE, bedwetting associated with diurnal urinary loss, squatting and urge incontinence) were reviewed. The children underwent urodynamic studies to detect detrusor instability and the prevalence was compared with the type of enuresis. RESULTS Of 33 patients with MPE, 17 (49%) showed either typical unstable detrusor contractions (16) or low-compliance bladders (one); in the remaining 16 patients, filling cystometry was normal and micturition was normal in all. Of the 47 patients with CPE, 35 (79%) showed detrusor instability and two decreased bladder compliance; the remaining 10 had stable bladders and micturition was also normal in all patients. CONCLUSIONS The type of primary enuresis and the maximum cystometric bladder capacity were good indicators of bladder dysfunction.