These cysts are apparently quite rare. The author gives histological and embryological discussions, mentions the age and sex incidence, symptoms, and pathological anatomy, as obtained from the literature. The clinical history of a case is reported in which the affected kidney was surgically removed. Hydronephrosis, chronic pyelitis and ureteritis were also present. The kidney measured 12X8X7 cm., and contained a large cyst 8X6 cm. with calcareous walls enclosing yellow fluid. It lay wholly within the fibrous capsule of the kidney and had no connection with the pelvis. The interior surface of the cyst was yellowish white and narrow ridges projected from the surface, suggesting former septa; there were no other gross lesions. The pelvis was dilated and chronically inflamed. Microscopic examination showed bundles of connective tissue with small areas of necrosis and calcification near the surface; no epithelial lining; many cholesterin crystals in the fluid content; and a few hyaline glomeruli surrounded by new connective tissue and mononuclear round cell infiltration. It may be an old hematoma, an old abscess cavity or a congenital cyst. The calcification is the unusual finding.