The association of nephronophthisis and tapeto-retional degeneration was described by both Senior and Loken in 1961, but prior to 1974 only 28 cases had been published. This report describes 8 new cases in 27 members of 5 families. The severe juvenile type produces blindness in infancy and death from renal failure before the age of ten. The adult type is characterized by later onset, slower progression of the renal disease and milder ocular manifestations. The eye disease may be congenital amaurosis of Leber type, pigmentary retinal degeneration or retinitis punctata albescens and the electroretinogram (ERG) is of value in the diagnosis of these varieties of hereditary tapeto-retinal degeneration. Renal involvement is often asymptomatic. Defective urinary concentration leading to polyuria and polydipsia is the earliest sign. Proteinuria is inconstant and urinary sediment is often normal. Two patients had aminoaciduria. The disease progresses inexorably to chronic renal failure. One patient has been successfully transplanted and two others are on chronic hemodialysis. Renal histological changes are those of nephronophthisis with tubulointerstitial lesions and multiple cysts. Senior-Loken syndrome appears to be transmitted by a single autosomal recessive pleotropic gene of variable expression. Degeneration of neuroepithelium and renal tubular epithelium, both tissues of ectodermal origin, may represent a genetically determined enzyme abnormality.