Congenital myotonic dystrophy (CMyD) affects the brain, causing mental changes and psychomotor retardation. However, the pathophysiology of the brain dysfunctions in CMyD remain to be clarified. We described two cases of CMyD with brain abnormalities. Case 1 was diagnosed as having ventricular dilation at 17 days after birth, and died at 3 years and 6 months. Case 2 was diagnosed as having ventricular dilation at birth, and died at 1 year and 3 months. Pathologically, both cases showed remote hypoxic ischemic brain damage and leptomeningeal glioneuronal heterotopia (LGH). In our patients, the white matter changes may have been caused by perinatal asphyxia, and LGH by embryological abnormalities. Taken our data and those of previous reports together, it is suggested that cerebral abnormalities in CMyD are ascribed to both hypoxic ischemic changes and histogenetic abnormalities.