Radioisotope scintillation scanning has been applied to the diagnostic problem presented by thoraco-abdominal masses in the midline. The technique consists of the intravenous injection of human serum albumin iodinated with the radioactive isotope I131and the use of a scintillation-scanning apparatus to reveal the distribution of the tracer-tagged blood pools within the thorax and upper abdomen. Study is facilitated by superposing the transparent record given by the scintillation-scanning apparatus upon the matching chest roentgenogram. The cases of 4 patients here described illustrate the safety and usefulness of this method as a substitute for contrast roentgen aortography in the differential diagnosis between midline solid tumors and aneurysms of the great vessels.