The ultrastructual localization of thorium dioxide was examined in liver biopsy specimens from two patients injected more than 30 years before with thorotrast. An energy dispersive X-ray microanalysis spectrometer (Kevex 5100) was used to identify thorium in the liver tissue. By electron microscopy, most of the thorium particles were found in the cells of the reticuloendothelial system, such as macrophages of the portal triad and Kupffer cells in the hepatic sinusoid. Thorium dioxide particles were mainly located within the phagosomes, but larger aggregates existed in the cytoplasma with no visible limiting membrane. In addition, the deposition of thorium granules in a few hepatocytes was also confirmed by electron microscopy. This fact indicates that two main pathways of elimination are still functioning in the late period, but the hepatocytic pathway appears to be less effective than the reticuloendothelial system.