Urinary N-acetyl-beta-D-glucosaminidase (NAG) is excreted from renal tubular cells and increased urinary excretion reflects renal tubular damage. There are many causes for the elevation of urinary NAG; however, primary elevation of urinary NAG (primary hyper-NAGuria) has never been reported. Recently, we encountered two siblings with continuously elevated urinary NAG levels that could not been explained by any known causes. Two brothers, aged 13 and 11 years, were found to have continuously elevated urinary NAG levels that were repeatedly confirmed by urinary NAG levels (both per liter and urinary NAG/creatinine ratios). However, simultaneously measured urinary beta(2)-microglobulin levels were always within normal ranges. The elder brother is a heterozygous carrier for familial hypercholesterolemia, although this was not attributable to the urinary NAG elevation. The two patients were receiving no drugs when they were first examined at our hospital, and no nephrotoxic substances were found to be the cause for the elevation. Renal biopsy revealed no abnormal findings attributable to the abnormally high excretion of urinary NAG. Our two patients are considered to have asymptomatic primary hyper-NAGuria, which has not been previously reported. Although more cases are needed, this asymptomatic primary hyper-NAGuria is probably a new clinical entity of renal tubular disorders.