Brain autopsy materials from 5 patients diagnosed clinicopathologically as having myotonic dystrophy (MyD) were studied. All of the cases showed many rod-like intracytoplasmic inclusions, which have never been reported in the disease, in the large neurons of the caudate nucleus. The rod-like inclusions were observed in the cytoplasm near the nucleus, and showed eosinophilic staining with HE, blue with PTAH, red with Masson's trichrome, and blue with toluidine blue. Electron microscopically, the inclusions had an internal structure composed of parallel beaded dark filaments and clear spaces. In transverse section, the inclusions showed tetragonal arrays of filaments. These histological characteristics of the inclusions were similar to those of inclusions reported previously in the same location in various disorders, showing increasing incidence with aging. Quantitative investigation on the 5 patients and 24 control subjects revealed that the number of neurons containing the inclusion in the caudate nucleus of MyD patients was about twice as many as the control subjects, and that the occurrence rate of the inclusion body to the large neurons in the caudate nucleus in MyD was far higher than that of the controls. The finding indicates that the inclusion body is important for the diagnosis of the disease, and may have a close relationship with the neurological symptoms and pathogenesis of the disease.