Six families including 27 patients with adult nephronophthisis were studied. The diagnosis was based on the evidence of heredofamilial chronic tubulointerstitial renal disease, not related to urologic abnormality, and progressing to renal failure in at least 2 members of each kindred. In two families, the mode of inheritance is compatible with recessive autosomal transmission; retinal heredodegeneration was found in both kindreds; mean age at end-stage renal failure (ESRF) was 22 years. In one additional case, kidney involvement is apparently sporadic, and associated with retinal changes and blindness. In one kindred, the mode of inheritance could not be determined. In the last 3 families, pedigrees are compatible with dominant autosomal transmission; retinal involvement was found in none; ESRF developed at a mean age of 47 years. Renal cysts were detected by ultrasonography in 9 of 12 patients; they were located at the cortico-medullary junction or in the medulla in 5 cases. Renal biopsy showed rather similar changes in the various kindreds. These results are compared with those already reported from 29 families with adult nephronophthisis.