We examined urinary N-acetyl-beta-D-glucosaminidase (NAG) levels in 84 children with grade III to grade V primary vesicoureteric reflux and in 31 controls. Age and sex of patients were similar in controls and patients and none had evidence of urinary infection at the time of testing. The mean urinary NAG activity levels in non-refluxing controls was 4.98 IU/g Cr, standard error (SE) 0.756. The mean urinary NAG activity levels in grade III patients (n = 43) was 6.024 IU/g Cr (SE 0.898), in grade IV (n = 28) 9.059 IU/g Cr (SE 1.317), and in grade V (n = 13) 17.298 IU/g Cr (SE 5.124). In this study, only patients with grade IV and V vesicoureteric reflux demonstrated significantly elevated NAG levels and within these grades of reflux, high NAG activity was confined to those patients who had associated renal scarring. NAG estimation in these patients with reflux nephropathy may be a useful simple additional test to assess tubular damage.