Thorium dioxide (Thorotrast), a radioactive substance with a very long half-life, has been used in the past as a contrast medium for various roentgenologic studies. Some of these have been followed by the delayed appearance of benign and malignant tumors, especially of the liver. Chronic arachnoiditis and myelopathy developed in three patients after myelography with thorium dioxide. Progressive involvement of the cauda equina, spinal cord, and brain stem was observed. One patient died after 26 years of illness.