Human herpesvirus 6 (HHV-6) infected cells were detected in all lung, lymph-node, spleen, liver, and kidney tissues obtained at necropsy from an unselected series of nine patients with AIDS. This infection rate was significantly higher than that for cytomegalovirus. Lung infection with HHV-6 was extensive enough in one patient to account for fatal pneumonitis. In other tissues increased numbers of HHV-6 infected cells were related to the presence of lymphocytes infiltrates or residual lymphoid tissue, with lymphocytes being predominantly infected. Thus HHV-6 is an important pathogen in patients with AIDS.