The pattern of urinary protein excretion was followed for 2 to 19 months in 15 children with the congenital nephrotic syndrome of the Finnish type. The duration of the disease and the renal histopathological changes were correlated with the urinary total protein and albumin excretion, the sieving coefficients of five proteins, the selectivity angle based on relative clearances of four proteins, and the excretion of beta-2-microglobulin. Total urinary protein excretion increased with time; in general, the proteinuria was of glomerular type and highly selective. With advancing histological lesions selectivity declined corresponding to the increases in individual sieving coefficients, and there were signs of secondary tubular impairment as shown by increased beta-2-microglobulin excretion. Of the histopathological changes, tubular atrophy correlated best with the various measures of proteinuria. The findings support the concept of a primary glomerular disease with secondary tubular injury.