The quantitative histopathological analysis was performed on 68 renal specimens of 50 children with congenital nephrotic syndrome of Finnish type (CNF). Thirty five of samples were biopsies and 33 were taken at autopsy. The kidneys of the CNF children were 2-3 times larger by weight than those of normal children. The renal histological picture varied greatly from one patient to another. The glomeruli of the CNF patients showed more often fibrotic lesions and mesangial hypercellularity than those of the normal infants, while immature glomeruli were seen more frequently in the controls than in CNF children. Tubular atrophy and tubular dilatations were also common histological feature in the CNF children. Glomerular fibrosis and tubulo-interstitial changes showed significant progression during the course of the disease while mesangial cell proliferation did not increase with age. The decreased tubular cell mass was replaced by interstitial tissue in the kidneys of older CNF infants. The proportion of the total glomerular volume to the volume of the whole kidney was about the same in CNF as in normal children. Because of the greater kidney size in CNF the absolute volume of the total glomerular mass is increased in this disease.