Urinary albumin and β2-microglobulin (B2M) were measured during the neonatal period. Urinary albumin decreased postnatally in term neonates, while it remained almost constant in preterm neonates. Urinary B2M showed a peak level on day 7 both in term and preterm neonates. There was some trend towards higher levels of albumin and B2M with decreasing gestation, showing that glomerular permeability increases and proximal tubular protein reabsorption decreases with increasing degrees of prematurity. In sick preterms who were depressed at birth and had respiratory failure, both parameters were elevated during the first 2 weeks, indicating the presence of glomerular and tubular damage in this period. The changes in B2M with gestation or clinical condition were more pronounced than those in albumin