Electron microscopic examination was carried out on 10 glomeruli from 4 infants with congenital nephrotic syndrome of the Finnish type (CNF) and 5 glomeruli from 1 control. The thickness of the peripheral glomerular basement membrane (GBM) and lamina densa were measured. The width of the GBM was thinner in CNF than in the control but still within normal limits for the age and the width of the lamina densa was thinner in CNF compared to the control. This study supports earlier findings that the basic defect in CNF could be a faulty synthesis of the lamina densa.