Human autosomal dominant polycystic kidney disease (ADPKD) is the most common lethal dominant hereditary disorder characterized by enormous renal enlargement and the development of multiple cysts originating from nephrons. We investigated the pathogenesis of cyst formation in ADPKD by using patch-clamp and immunocytochemical techniques. Adenosine 3',5'-cyclic monophosphate-activated Cl- currents are present in primary cultures of ADPKD cells and have characteristics such as a linear current-voltage relation, insensitivity to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, sensitivity to glibenclamide and diphenylamine carboxylic acid, and an anion selectivity sequence of Br- > Cl- > I- > glutamate, all of which are identical to cystic fibrosis transmembrane conductance regulator (CFTR). With the use of CFTR antibodies raised against the regulatory and first nucleotide-binding domains, CFTR was detected in primary cultures of ADPKD cells. Similar results were obtained in vivo in cyst-lining epithelial cells in ADPKD kidneys, where staining was seen associated with the apical membrane regions. These data indicate that the CFTR Cl- channel exists in apical membranes of ADPKD cells and may play an important role in cyst formation or enlargement.