The value of peritoneoscopy in the diagnosis of renal polycystic disease was tested in nine patients with adult polycystic disease and one with congenital hepatic fibrosis. The right kidney was visualized in all ten cases and the left in seven. Renal cysts were recognized by peritoneoscopy in eight of the ten patients. Factors impeding visualization of the kidneys and the cysts were: a) fixation of the splenic flexure of the colon; b) non-superficial renal cysts. Of six cases in which intravenous pyelography was not diagnostic, peritoneoscopy was positive in four. No correlation was found between the degree of hepatic and renal cystic involvement.