Evaluation of fetal renal function by analysis of fetal urine sampled in utero may improve perinatal care after a prenatal diagnosis of bilateral obstructive uropathy. We provide reference intervals for 10 fetal urinary compounds and examine their variation with gestational age. Forty-one fetuses with bilateral obstructive uropathy (urine sampled between 20 and 38 weeks of gestational age) had normal, healthy values for serum creatinine (< or = 50 mumol/L) at ages 1-2 years. These cases were thus assumed to represent a reasonable approximation to healthy values. Sodium and beta 2-microglobulin concentrations significantly decreased with gestational age; calcium, ammonia, and creatinine significantly increased; glucose, phosphorus, chloride, urea, and total protein concentrations did not vary. Our results provide reference values for prenatal evaluation of fetal renal function and suggest that glomerular filtration of macromolecules and tubular reabsorption of glucose and phosphorus are mature by 20 weeks of gestation, whereas tubular reabsorption of sodium and beta 2-microglobulin increases progressively during the second half of gestation.