We studied the kidneys from ten patients with adult (autosomal dominant) polycystic kidney disease (APKD) stained with lectins specific for different segments of the nephron on 20 cysts from each case (ranging in size from 0.1 to 1.3 cm in nine cases and from 1.5 to 6 cm in one case). The epithelium of all cysts with positive reactivity (Arachis hypogaea and epithelial membrane antigen) was of collecting duct origin. Many cysts remained unstained. Cysts of proximal tubule origin could not be identified using the specific lectin Lotus tetragonolobus. Focal epithelial hyperplasia appeared in the collecting duct cysts. Cysts surrounded by smooth muscle were frequent and considered to be of collecting duct origin. One case had glomerular cysts. We conclude that the cysts of APKD are principally of collecting duct origin.