This study was performed to clarify the changes in urinary albumin excretion according to age and sex in healthy subjects, for the appropriate judgment of microalbuminuria in insulin-dependent diabetes mellitus (IDDM). Concentrations of albumin in the first morning urine from 2990 healthy individuals (1713 males, 1277 females) aged from 6 to 39 years were measured by turbidimetry immunoassay. Reference values of the concentrations for each age group were obtained from the 95% confidence limits from the distribution simulated by the chi 2-test (k = 1). Reference values were 1.3-2.2 mg/dl in children aged 6-11 years. Values for females increased significantly from 22 mg/dl at age 11 years to 3.9 mg/dl at age 12 years (P < 0.0001) and those of males increased gradually from 1.7 mg/dl at age 11 years to 3.9 mg/dl at age 16 years. There were significant differences between males and females at the ages of 12 (P < 0.0001), 13 (P < 0.0001), and 14 (P < 0.0007). After 17 years, the values decreased from 3.9 mg/dl to 3.3 mg/dl in females, and to 2.9 mg/dl in males. We conclude that reference values of urinary albumin excretion change according to age and sex, which should be taken into consideration in the assessment of diabetic nephropathy in IDDM.