To compare and characterize the magnetic resonance imaging (MRI) of brain in the congenital and adult form of myotonic dystrophy type 1, we evaluated five patients with congenital dystrophy type 1, 10 age- and 10 disease duration-matched patients with adult-form dystrophy type 1 and 20 age-matched healthy volunteers. The ventricular enlargement was evaluated by the ventricular:brain ratio, the signal intensity of white matter posterosuperior to trigones by reference to standard images and the white matter lesions by a semiquantitative method. In the congenital dystrophy type 1, MRI was characterized by ventriculomegaly and moderate/severe hyperintensity of white matter posterosuperior to trigones, which showed no correlation with the age. MRI in the adult-form dystrophy type 1 was strictly related to disease duration and varied between normal findings, except for temporo-polar white matter lesions, in age-matched patients and ventriculomegaly with white matter hyperintensities in disease duration-matched patients. These results suggest that the origin of MRI abnormalities in myotonic dystrophy type I is mainly developmental for the congenital form and mainly degenerative for the adult form. (C) 2002 Elsevier Science B.V. All rights reserved.