Urinary excretion of N-acetyl-beta-D-glucosaminidase (NAG) was studied in 22 children with proteinuria and a normal glomerular filtration rate. Twelve patients had minimal change nephrotic syndrome (MCNS), 10 children suffered from different types of glomerulonephritis (GN) without tubulointerstitial renal disease. Sixty-six random urine samples were analyzed for protein, creatinine and NAG. There was a strong positive correlation between proteinuria and urinary NAG activity (r = 0.79, p = 0.0001). The raised NAG activities in proteinuric states turned to normal during remission of the disease. Patients with MCNS and GN did not differ significantly and had nearly identical correlation curves. Our results seem to indicate that functional changes in the renal tubular cells probably caused by protein resorption are responsible for this association. Raised NAG activities in proteinuria reflect the activity of the disease but cannot distinguish patients with MCNS and GN.