The distribution of thorium in the liver of a patient 36 y after injection with Thorotrast was examined with autoradiographic and scanning electron microscope backscatter image techniques. Autoradiographic examination of randomly selected histologic sections of the liver showed a total alpha activity calculated at 33.7 Bq g-1, with the highest concentration of alpha activity sequestered in subcapsular scare tissue. Subcapsular scare tissue received 4.8 cGy d-1 of alpha radiation, periportal areas were accumulating 1.4 cGy d-1, and the hepatic cord areas 0.09 cGy d-1 of alpha radiation at the time of death. The concentration of dose in periportal areas correlates with higher incidence of bile duct tumors (than hepatocellular carcinomas) found in patients exposed to Thorotrast. The backscatter technique was demonstrated as useful for identifying thorium in liver specimens.