Myotonic dystrophy type1 (DM1) is a multisystemic disorder caused by a CTG repeat expansion in the DMPK gene. Aberrant mRNA splicing of several genes has been reported to contribute to some of the symptoms, including myotonia and insulin resistance, but the cause of muscle wasting is unknown. Dystrophin is a cytoskeletal protein that is required for structural stability and signaling at the sarcolemma and has several spliced isoforms. We investigated the alternative splicing of dystrophin in skeletal and cardiac muscle of DM1 patients by using reverse transcriptase-polymerase chain reaction and found that dystrophin isoforms lacking exon 71 or 78, which is suggested to encode an important region for protein binding and hydrophobicity, were significantly increased. We suggest that the aberrantly spliced dystrophin is responsible for the muscle wasting in DM1.