Few data have been collected on embryonic neoplasms of the kidney. Comparatively speaking, they are rare in man and the lower animals. In human beings the condition is usually spoken of as Wilms' tumor, and it occurs almost exclusively in children. In the lower animals it has been reported as occurring in rabbits, swine, and chickens. A larger number of embryonic renal neoplasms has been reported as occurring in swine than in all other animals combined, and from this one might infer that the tumor is more common in swine than in other species. Day reported eight cases occurring in swine and later Feldman added a series of eleven cases. The five cases found in rabbits were reported by Scott, Polson, and Bell and Henrici. Mathews reported a series of twelve cases in chickens. The two cases constituting the basis of this report occurred in a flock of 400 white leghorn hens, and both birds came to necropsy on the same day.