Disseminated cytomegalovirus (CMV) has been investigated by in situ hybridization in formalin-fixed paraffin-embedded tissue sections with biotinylated DNA probes. Two cases of disseminated CMV infection were studied at autopsy by this highly specific technique. The presence of CMV in cytomegalic cells is readily shown. In addition, CMV has been detected and localized in many normal-appearing cells. This occult infection occurs in cardiac myocytes, hepatocytes, spleen and lymph node reticular cells, endometrial stromal and glandular cells, and breast stromal cells, as well as in cells in the renal glomerulus, tubule, and interstitium, adrenal cortex and medulla, fallopian tube submucosa, myometrium, and anterior pituitary. Cytomegalovirus infection of endothelial cells has been further documented by immunohistochemical methods utilizing antibody to Factor VIII. These findings suggest that CMV disseminates hematogenously throughout the body, initiating necrotizing foci of infection. The appearance of many diffuse foci suggests that local viral spread occurs via endothelial cell infection. Surprisingly , lymphocyte involvement was not observed.