We describe an anal disease activity index suitable for use in prospective studies of treatment and recording the natural history of anal disease. We studied 40 patients with perianal Crohn's disease (PACD), 14 patients with anal disease not related to Crohn's disease, and 10 normal individuals. Seven symptoms related to anal disease were measured using a linear analog scoring system, which proved easy for the patients to complete. Only three of the seven clinical parameters studied before and after treatment had a high discriminant value. On the basis of these findings, we conclude that a good index of response to therapy in patients with anal disease can be obtained from a linear analog scoring of three symptoms: spontaneous anal pain, pain following defecation, and inhibition of locomotion by pain. The index should be of value in comparing management options in PACD.