In this interesting and unusual case of coexisting primary osteosarcoma of the pelvis and primary carcinoma of the urinary bladder, the bone tumor was originally considered benign on the basis of history, clinical examination, and examination of the gross specimen after operation. Rapid recurrence took place and caused much distress because of the situation and extent of the tumor. Another primary tumor, a highly malignant and rapidly growing carcinoma of the bladder; brought about a fatal termination two weeks after the first symptoms referable to the urinary tract were noted.