A 33 year-old Thai woman was diagnosed with scrub typhus infection according to clinical symptoms, eschar lesions compatible with the disease, and specific antibody to Rickettsia tsutsugamushi detected by indirect immunoperoxidase. Percutaneous transhepatic needle biopsies were taken before and 7 days after treatment with tetracycline to study the pathology of the liver. The liver tissue was evaluated by light microscopy, using H & E and Pinkerton's stains, and by transmission electron microscopy (TEM). Before treatment it showed reactive hepatitis.Rickettsia organisms within the hepatocytes and sinusoids detected by Pinkerton's stain appeared as tiny bright-red organisms. By TEM, the rod-shaped double-membrane Rickettsiae appeared intact in the cytoplasm of Kupffer's cells and hepatocytes. After tetracycline treatment, moderate levels of acidophilic and ballooning liver cells were observed. The degree of cytoplasmic organelle damage varied, including fatty metamorphosis, depletion of glycogen granules, loss of the mitochondrial cristae, dilatation of endoplasmic reticulum and cytoplasmic vacuolation. Rickettsia organisms cannot be visualized by Pinkerton's stain but were detected by TEM, in markedly vacuolated hepatocytes, in congested sinusoids and in Kupffer's cells. Intranuclear Rickettsia were discovered in the endothelial nucleus, showing various degrees of injury. Some were mildly degenerated, while others exhibited clumping of nucleoprotein at the cytoplasm periphery and large vacuolation centrally. Many indented organisms were found, and binary fission during Rickettsiae multiplication was always affected. Electron-microscopic examination of hepatic injury associated with scrub typhus is rare. This is the first ultrastructural localization ofRickettsiae in the infected human liver.