To evaluate a magnetic resonance imaging (MRI)-based score of disease severity for perianal fistulizing Crohn's disease (CD), 16 patients with perianal fistulizing CD underwent 1.5-T MRI before and after infliximab therapy. MRI scores, Perianal Disease Activity Index (PDAI) and C-reactive protein (CRP) were calculated, and fistula drainage was assessed. These items were compared for responders and nonresponders. After therapy, PDAI values and CRP values decreased significantly in responders, but there were no significant differences in the MRI scores. In all responders, MRI findings were indicative of persisting active inflammation. In these patients, a relapse rate of 57% was seen.