Nephronophthisis (previously described as familial juvenile nephronophthisis and medullary cystic disease) is characterized by insidious renal failure, its main features being increased urinary sodium loss, pitressin-resistant hypotonic polyuria, polydipsia, normal urine sediment and absence of hypertension. Renal function and histologic studies were performed in a family in which two siblings had this disorder, while the parents and two other siblings appeared clinically normal. Both parents demonstrated a moderate impairment of maximum urinary concentration. The values for tubular free water reabsorption (TcH2O) were relatively normal in the parents and the healthy siblings. One of the index patients showed only minimal sodium wasting even though he had hyposthenuria, thus suggesting an involvement of the collecting ducts in the early stage of nephronophthisis. No evidence of proximal tubular dysfunction was found. Although the light-microscopic examination of renal biopsies from the parents and the healthy siblings was unremarkable, electron microscopy revealed probable abnormalities in all four. An autosomal recessive mode of inheritance is, therefore, suggested in this family. The etiology of nephronophthisis is obscure but a likely possibility is that the renal damage results from an inborn metabolic error.