Thorotrast is a radiopaque medium widely employed in recent years, notably in angiography. Its use, however, is followed at times by damaging effects on tissues which become apparent in years rather than months, stemming most frequently from its accidental perivascular deposition (3). Rarely, its deposition within the cells of the reticulo-endothelial system leads to aplastic anemia (10, 16, 27) and neoplastic growth (16). In both instances, the effects are the result of the radioactive properties of the thorium salt. Thorotrast is a 25 per cent solution, by volume, of colloidal thorium dioxide in a dextrin medium. The element thorium contained in this preparation is the parent substance of a series of compounds constituting one of the three naturally occurring radioactive families, the others being radium and actinium. It has an estimated half-life of 1.34 billion years (13). The radioactive decay of the thorium series releases alpha particles, beta rays, and gamma rays. Biological effects are due chiefl...