Sporadic inclusion body myositis is a disease of unknown pathogenesis in which a viral etiology has long been suspected. We report a case that occurred in a patient with human T cell leukemia virus type 1-associated myelopathy. The diagnosis was confirmed by histopathological studies of the deltoid muscle. Nucleic acids amplification and in situ hybridization indicated the presence of integrated proviral DNA and viral mRNA transcripts in the lesions.