Urinary N-acetyl-beta-D-glucosaminidase (NAG) was determined in infants during the first year of life to clarify its evolutional change and the reliability of this enzyme as a marker of renal tubular damage in the newborn period. During the first 2 weeks of life, there was no difference in the urinary levels between preterm (gestational age of 33-36 weeks) and full-term infants. The urinary NAG index (NAG is expressed as the ratio of the enzyme activity to milligrams of creatinine) during the first month of life was significantly elevated when compared with that of any other succeeding period and gradually decreased during the first year of life. The index in newborn infants on tobramycin was significantly elevated and maintained a higher value during the 5 days after withdrawal of the medication. Infants with asphyxia had a higher urinary NAG index during the first week of life. These data suggest that the urinary NAG index is a useful indicator of renal tubular damage during the newborn period.