The extent of disease as detected by magnetic resonance imaging was compared between 32 patients with benign multiple sclerosis (MS) and 32 patients with the chronic progressive form matched for age, sex, and disease duration. Computer-assisted quantitation of magnetic resonance images revealed a higher mean lesion load in chronic progressive multiple sclerosis (CPMS); however, in approximately 20% of benign MS patients the lesion load was higher than that in the CPMS patients. CPMS patients had a higher number of infratentorial lesions, yet similar numbers of supratentorial lesions, when compared with benign MS patients. The degree of confluency of lesions and the clinical expression of infratentorial lesions were typically higher in the CPMS patients. Benign MS was characterized by a lower degree of confluency and a higher number of asymptomatic infratentorial lesions. Thus, magnetic resonance imaging shows characteristic differences in magnetic resonance-detected changes between MS patients with different clinical courses.