A light microscopic study was performed on 10 fetal kidneys of congenital nephrosis of the Finnish type (CNF) and compared to 22 controls. The gestational age of the fetuses ranged from 16 to 24 weeks. A quantitative morphometric analysis was carried out on 6 CNF fetuses and 6 age-matched controls. The organogenesis of the kidney and the nephrogenesis proceeded normally in CNF. The size and the number of glomeruli were the same in CNF and controls. An increase in the proportion of mature glomeruli and a decrease in the percentage of visceral epithelial cells in them suggested accelerated glomerular maturation in CNF. Focal mesangial cell hyperplasia and dilatation of glomerular capillaries with congestion of blood were more often obvious in CNF than in controls. Marked dilatation of proximal tubular lumens and Bowman's spaces were typical in CNF.