An autopsy case of dermatomyositis (DM) associated with interstitial pneumonia probably due to cytomegalovirus infection is reported. After corticosteroid therapy for 1 month, a 79-year-old man with DM developed acute respiratory failure due to interstitial pneumonia and died in spite of intensive respiratory care. By polymerase chain reaction method (PCR), DNA of cytomegalovirus (CMV) was detected in the bronchoalveolar lavage fluid (BALF). CMV was also detected by the method of conventional virus culture from BALF. These findings suggested that initial infection or reactivation of CMV had occurred in the lungs. The autopsy specimen revealed the findings of interstitial pneumonia compatible with CMV pneumonitis, but without the presence of intranuclear inclusion bodies. Although the present case of interstitial pneumonia should not strictly be diagnosed as definite CMV pneumonitis without the presence of intranuclear inclusion bodies in the lung tissue, initial infection or reactivation of CMV in the lungs may have contributed to the pathogenesis of interstitial pneumonia. In other cases of collagen vascular disease associated with interstitial pneumonia, CMV or other viruses may contribute to the pathogenesis of interstitial pneumonia.