A 2-year-old neutered male cat with idiopathic hemolytic anemia had cartilaginous exostoses on the ribs, sternum, vertebrae and scapulae. The range of lesions in the cat resembled that reported in the dog. Radiographic and histologic appearances of the cartilaginous exostoses were like those reported for other species. Theories of the pathogenesis of this disease are presented. Electron microscopic examination of two widely separated cartilage-capped exostoses showed ‘C'-type viral particles resembling those of feline leukemia to be associated with the plasma membrane of chondroblastic elements.