We report a 53‐year‐old Jamaican man with 20 years of progressing weakness involving proximal limb muscles and neck flexors. Serum CK was 1100 IU/L. EMG demonstrated spontaneous activity, myopathic motor units, and full recruitment patterns in weak muscles. Muscle biopsy revealed marked myofiber degeneration with extensive fibrosis, suggesting a chronic myopathic process. HTLV‐1 antibody was present in serum in high titers by ELISA and Western blot. Immunohistochemistry with rabbit polyclonal antisera to HTLV‐1 showed rare staining myocytes. PCR demonstrated HTLV‐1 DNA in frozen muscle tissue. This myopathy, associated with HTLV‐1 infection, has clincal and pathological features similar to a dystrophy. We recommend serological screening for HTLV‐1 in cryptogenic adult myopathies. © 1993 John Wiley & Sons, Inc.