Polycystic kidney disease is a bilateral disorder that affects approximately 200,000-400,000 persons in the United States. The most common form of the disease is inherited as an autosomal dominant trait (ADPKD). It typically causes renal insufficiency by the fifth or sixth decade of life. The disease is characterized by the progressive enlargement of a portion of renal tubule segments (proximal, distal, loop of Henle, collecting duct). The tubules enlarge from a normal diameter of 40 microns to several centimeters in diameter, causing marked gross and microscopic anatomic distortion. The cause of the cystic change in the tubules is unknown, but current possibilities include obstruction of tubule fluid flow by hyperplastic tubule cells, increased compliance of the tubule basement membranes, and/or increased radial growth of cells in specific portions of the renal tubule. Several studies show that the epithelia of the cysts continue to transport Na+, K+, Cl-, H+, and organic cations and anions in a qualitative fashion similar to that of the tubule segment from which they were derived. ADPKD, then, is a disease in which some gigantic renal tubules, over a period of several decades, impair the function of nonaffected nephrons and thereby lead to renal failure.