Magnetic resonance imaging scans of the brain were obtained in 13 patients with myotonic dystrophy, seven with congenital myotonic dystrophy and six with adult-type myotonic dystrophy. All seven patients with congenital myotonic dystrophy had ventriculomegaly and a low IQ (DQ). Cerebral white matter lesions were observed in six cases, a small corpus callosum in four cases, a small brainstem in two cases, and a cerebellar white matter lesion in one case. Cerebral white matter lesions were observed in five of the six cases with adult-type myotonic dystrophy of which one had ventriculomegaly. The IQ (DQ) was significantly lower in patients with congenital myotonic dystrophy than in those with adult-type myotonic dystrophy. The incidence of a small corpus callosum or ventricular enlargement was higher in congenital myotonic dystrophy than in adult-type myotonic dystrophy. These findings may be related to the presence of neurologic impairment in congenital myotonic dystrophy.