We reviewed the brain MRI of 66 patients with the adult form of myotonic dystrophy type 1 (DM1) to evaluate the extent and significance of white matter involvement and to look for a pattern of MRI abnormalities suggestive of DM1. White matter lesions (WMLs) and large Virchow Robin spaces (VRSs) were rated by semiquantitative methods and the signal intensity of white matter superior and posterior to the trigones (WMPST) by reference to standard images. Disease duration was correlated positively with WML and negatively with VRS scores. Patients were divided into four groups according to increasing severity of WMPST involvement: group A with mild WMPST hyperintensity, group B with large VRSs and mild or moderate WMPST hyperintensity, group C with moderate WMPST hyperintensity or mild WMPST hyperintensity with small WMLs, group D with severe WMPST hyperintensity or moderate WMPST hyperintensity with small WMLs. Disease duration, muscular impairment, lobar WMLs and brain atrophy significantly increased from groups A and B (not significantly different) to C and from C to D, while convexity VRSs significantly decreased from group B to C and from C to D. Lobar white matter involvement in DM1 seems progressive during the disease and may be characterized initially by large VRSs or mild WMPST hyperintensity, then by small WMLs or moderate WMPST hyperintensity, and finally by more extensive and confluent WMLs or diffuse white matter hyperintensity and by brain atrophy.