TUMORS arising in a horseshoe kidney are relatively rare. Hildebrand1in 1895 is credited with reporting the first case, and since then a review of the world literature through December 1966 including our two patients has disclosed a total of 71 additional cases of cancer in a horseshoe kidney.2-63 It seems cancer is no more likely to occur in a horseshoe kidney than it is in a "normal" kidney. According to Glenn64, a consensus of the various estimates on the incidence of horseshoe kidney in the general population indicates that this anomaly occurs once in every 400 to 500 births. The total population of the United States including the armed forces overseas as reported in the 1960 census was approximately 180 million people.65If as few as one in every 1,000 persons has a horseshoe kidney, then in 1960 there were about 180,000 horseshoe kidneys in