Cyst formation and growth in autosomal dominant polycystic kidney disease. Previous morphologic studies on kidneys from adult patients with autosomal dominant polycystic kidney disease (ADPKD) indicates that the cysts developed from nephrons and collecting ducts in association with hyperplasia of epithelial cells lining the cyst walls. In the present study, we systematically evaluated by scanning electron microscopy 387 cysts in polycystic kidneys obtained from 10 adult patients. Some cysts were lined by cells typical of collecting duct (7.2%), proximal tubule (1.8%) or glomerular visceral (2.1%) epithelium. The remaining cysts were lined by a single layer of phenotypically undefined (84.0%) or markedly hyperplastic (4.9%) epithelium. The median plane surface area of individual cells within cysts was 182 × 10^-8 cm². Among all cysts the surface area of single epithelial cells ranged between 22 and 2530 × 10^-8 cm², but within single cysts the range of individual cell surface areas was more narrow. Mean cell surface area did not increase in direct proportion to cyst diameter; thus, epithelial hyperplasia is a central element in the progressive enlargement of cysts. In some cysts hyperplasia was accentuated by projections into cyst lumens of polyps, small adenomas and cord–like arrangements of cells. Epithelial polyps were found in the cysts of one non-azotemic patient, excluding renal failure as a cause of this accentuated type of epithelial proliferation. Morphologic evidence that cysts compressed adjacent renal parenchyma was observed in all kidneys. In 11 cysts bisected and then both halves thoroughly examined by scanning electron microscopy, one or two tubule openings were seen in one of the hemisections in three cysts (27.3%). The absence of tubule connections in 72.7% indicates that the intracavitary fluid probably derives from net transepithelial secretion in most cysts. On the basis of this study, we suggest that in ADPKD the major determinants of cyst enlargement are sustained epithelial hyperplasia and the sequestration of fluid delivered to cysts by transepithelial secretion and glomerular filtration.