Polycythemia, or more precisely, erythrocytosis, has been reported to occur with fibroids of the uterus,1 cerebellar hemangioblastoma,2,3 gastric carcinoma with metastases to the liver,4 hepatoma,5 prostatic carcinoma,6 and various renal lesions. In the latter group are hypernephroma,7-12 hydronephrosis,13,14 polycystic kidneys,7 adenoma,15 and fibromyxoma.7 That this relationship is more than coincidence is supported, first, by the statistically greater occurrence of these associated conditions than can be accounted for by chance alone and, second, by the observed improvement in the polycythemia after removal of the stimulating factor. The following case report introduces the possible relationship of bilateral unilocular cysts of the kidney with erythrocytosis. Report of Case The patient, a 65-year-old white man, was admitted to the Philadelphia Veterans Administration Hospital Aug. 15, 1958, with a history of leftsided hemiplegia of three weeks' duration. His past medical history revealed hypertension of a moderate degree. The physical examination revealed a 65-year-old, well-nourished,