To clarify the reliability of urinary trehalase activity as a marker of cellular proliferation and/or damage of renal proximal tubules, the activity was examined in healthy newborn infants or infants treated with tobramycin, a drug known as causing tubular cell damage. Eighty-one newborn infants (56 mature infants and 25 premature infants) were enrolled in the study. Urinary trehalase was examined using a spot urine sample during the first 7 days of age and on the 10th day of age. A good positive correlation was observed between urinary trehalase activity/creatinine ratio (T/Cr) on the 10th day of age and conceptional age or body weight (n = 46, r = 0.58, p < 0.001). Urinary trehalase of 29 healthy mature infants was higher during the first few days of age, after which it decreased to an almost steady level. Urinary trehalase of 6 premature infants during the first few days of age was significantly lower than that of mature infants, after which it increased and became equal to that of the mature infants on the 7th day of age. Treatment with ampicillin (100 mg/kg) and tobramycin (5 mg/kg) of 6 mature infants with pneumonia for 6 days resulted in a significant elevation of the urinary T/Cr. The extent of this elevation was greater than that of the urinary N-acetyl-beta-D-glucosaminidase (NAG) activity/creatinine ratio (NAG/Cr). A significant correlation was observed between the urinary T/Cr and the urinary NAG/Cr (r = 0.67, p < 0.01) or gamma-glutamyl transpeptidase/creatinine ratio (r = 0.48, p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)