A case of milk of calcium renal stone is reported. This is a rare disease in which a suspension of calcium salts is formed within a renal cyst. The pathognomonic sign is a fluid level seen in a standing position and an oval density seen in a supine position. In the present case, the milk of calcium was found to develop in a hydronephrotic kidney during the course of acute promyelocytic leukemia and this condition was suggested to be a complication of infection.