The origins of acquired cysts, hyperplastic epithelia of cyst walls, and renal cell carcinomas were investigated by evaluating their lectin conjugate reactivity. Paraffin-embedded blocks from 9 patients with acquired cystic disease were examined by the high-sensitivity lectin-antilectin immune peroxidase method. 11-176 lesions in each patient, 690 lesions in total, were stained both with Tetragonolobus lotus lectin (T) and peanut lectin (P); the former is specific for proximal tubules and the latter for distal tubules and collecting ducts. Out of 606 acquired cysts with single-layered epithelia, 559 (92.2%) were positive for T and negative for P, and 66 out of 75 (88.0%) cysts with hyperplastic multilayered epithelia were positive for T and negative for P. Three out of 4 solid adenomas and, to varying degrees, 5 renal carcinomas revealed the same reaction. These results suggest that almost all cysts accompanying acquired cystic disease of the kidney, including those with single-layered and multilayered epithelia, as well as solid adenomas and renal cell carcinomas, are derived from proximal tubules.