Subacute sclerosing panencephalitis, a rare, progressive, fatal central nervous system disease of children, is caused by measles virus. Clinical signs occur months to several years after recovery from acute measles infection. It is not known where the virus persists while the disease is inapparent. Involvement of organs outside the central nervous system has rarely been documented. To search for possible peripheral reservoirs of measles virus we used in situ hybridization to probe for measles virus RNA and immunocytochemical studies to localize measles virus antigens ina variety of organs taken at autopsy from confirmed cases of subacute sclerosing panencephalitis. Seven of 9 cadavers were found to contain measles virus RNA or antigens, or both, in at least one location outside the central nervous system. These sites included lymphoid organs such as thymus, spleen, lymph nodes, and tonsil, suggesting a role for lymphocytes in disease pathogenesis. Virus was also detected in kidney, lung, and glandular tissues such as pancreas, adrenal, and pituitary. These reservoirs may provide the antigenic stimulus leading to the elevated response characteristic for subacute sclerosing panencephalitis.