Light and electron microscopic as well as immunofluorescent findings in kidney biopsies of 9 patients suffering from nephronophthisis are reported. The lightmicroscopic picture is unspecific except the cysts at the cortico-medullary junction which are present in 70% of cases. The immunofluorescent findings are either negative or unspecifically positive. On electron microscopy, the tubular basement membranes exhibit characteristic but not specific changes, i.e. homogeneous thickening, splitting, reticulation, thinning, complete loss, granular disintegration and collapse. These findings suggest that the "Anlageschwäche" of the nephron especially of the tubular basement membrane may be the primary lesion in nephronophthisis.