PURPOSE To establish whether infants with vesico-ureteral reflux (VUR) have bladder dysfunction, with difference in gender, age at presentation and severity. PATIENTS AND METHODS 37 infants (24 male and 13 female) aged 2 to 24 months with II degrees to V degrees degree VUR underwent cystometry. Of those, 10 underwent natural filling cystometry. We considered: instability and maximal voiding detrusor pressure (VDP) to be "high" when it exceeded 90 cm H2O. We defined hypercontractility as high VDP and/or instability. RESULTS The prevalence of hypercontractility was 75% (18/24) in male and 46% (6/13) in female infants (p<0.004). High VDP was found in 50% (12/24) of male and 7% (1/13) of female patients (p < 0.001); no significant difference was found between male (25%) and female ones (38%) with instability alone. The mean VDP was significantly higher in male than in female infants (p < 0.001), in patients < 1 year of age than in older ones (p<0.001) and in severe than in moderate reflux (p<0.006). The mean voiding detrusor pressure of male infants was higher in severe (108+/-46cm H2O) and bilateral (101.3+/-44cm H2O) than in moderate (76+/-24 cm H2O) and unilateral (73.7+/-24 cm H2O) and in infants < 1 year of age (101.7+/-42 cm H2O) than in older ones (70.2+/-21 cm H2O). Natural filling cystometry confirmed the results of standard urodynamic studies. CONCLUSIONS Bladder dysfunction is confirmed also in infants with reflux, particularly in male younger patients, and it differs in gender. The pathogenesis of congenital reflux is not always a feature of malformation of the vesico-ureteral junction; therefore, patients with bladder dysfunction must be identified early.