Post-transplantation nephrosis in congenital nephrotic syndrome of the Finnish type. Congenital nephrotic syndrome of the Finnish type (CNF) is an autosomal recessively inherited disease manifesting as massive proteinuria, edema and ascites in the neonatal period. The disease is believed to be limited to the kidneys and recurrences after renal transplantation have not been reported. At our center 29 transplantations have been performed on 28 CNF patients. One to 33 months after transplantation, seven grafts (24%) of six patients have developed a steroid-resistant nephrotic syndrome. The clinical data and renal histology of these patients were analyzed in order to elucidate the cause of the proteinuria. At the onset of six of the seven episodes of nephrosis, the patient had evidence of a preceding CMV- or EBV-infection and the remaining patient had sinusitis. Upon light and electron microscopy examination, endothelial swelling of the glomerular capillaries resembling transplant glomerulopathy (TG) was seen, but unlike TG, the glomerular basement membranes were normal. The response of proteinuria to steroid or cyclophosphamide therapy was poor, with total remission in only two patients and partial remission in one patient, all treated with methylprednisolone and cyclophosphamide immediately after the diagnosis. Four grafts have been lost. Our data show that CNF patients have an increased tendency for post-transplantation nephrosis.