Twenty-one patients with nephronophthisis are described with a follow-up of one to 16 years (mean 9.3 years). In 10 patients, there was a familial incidence. Autosomal recessive appears the likely mode of inheritance with a 20 per cent incidence noted (seven of 35) following correction for the bias of ascertainment by removing the probands. Seven patients had an associated and characteristic retinal degeneration from infancy. Associated neurologic problems, including mental retardation, seizures and cerebellar ataxis, were also seen in some patients. Previously described skeletal abnormalities and hepatic fibrosis were not seen in any of our patients. All presented at an advanced stage of chronic renal failure, usually associated with a history of polydipsia and polyuria from infancy. Renal cysts were noted in only one of the nine patients in whom tissue was obtained by needle biopsy. In seven patients in whom tissue was available at nephrectomy or autopsy, cysts were noted in six although only in two were they localized to the medulla. Eighteen patients have undergone dialysis, and 12 patients have received a renal transplant with no evidence of recurrence of the original disease. Sixteen patients are still alive. Many synonyms for nephronophthisis have appeared, with medullary cystic disease being the most common. Our experience suggests that nephronophthisis is a common cause of chronic renal failure and has commonly associated nonrenal abnormalities.