During the last five years, we have observed four families in which siblings were affected with the congenital nephrotic syndrome (CN). Clinically, all patients, with one exception, could barely be differentiated from classical examples of CN of the Finnish Type (CNF). Morphologically, however, each family showed a different type of CN, although the appearance in siblings was identical. This morphological heterogeneity, makes it likely that CN in a population outside Finland is the result of an unknown number of autosomal recessive mutations at different gene loci. In an attempt to diagnose CN prenatally by means of measurements of the concentration of alpha-fetoprotein in amniotic fluid, we found two false normal results, one in a patient with histologically confirmed CNF, another in a patient with CN, histologically non Finnish Type. Thus the results of prenatal diagnostic procedures for these disorders have to be reevaluated.