Each of the three patients with a type of unilateral, localized cystic disease of the kidney that did not fit into a specific category of existing classifications had numerous cysts involving a part of one kidney. Selective angiography showed stretching of arteries around cysts that remained radiolucent in the nephrographic phase, and microscopic examination demonstrated that septa among the cysts contained compressed renal elements. The condition appeared to be neither progressive nor familial. All three had hypertension. Two patients who had nephrectomy for the cystic lesion became normotensive postnephrectomy. Despite some morphologic resemblances, this condition is believed to be different from adult type polycystic kidney disease in its clinical and genetic aspects.