Chronic anal fissure is a tear in the lower half of the anal canal that is maintained by contraction of the internal anal sphincter. Sphincterotomy, the most widely used treatment, is a surgical procedure that permanently weakens the internal sphincter and may lead to anal deformity and incontinence. We conducted a double-blind, placebo-controlled study of botulinum toxin for the treatment of chronic anal fissure in 30 consecutive symptomatic adults. All the patients received two injections (total volume, 0.4 ml) into the internal anal sphincter; the treated group (15 patients) received 20 U of botulinum toxin A, and the control group (15 patients) received saline. Success was defined as healing of the fissure (formation of a scar), and symptomatic improvement was defined as the presence of a persistent fissure without symptoms. After two months, 11 patients in the treated group and 2 in the control group had healed fissures (P=0.003); 13 in the treated group and 4 in the control group had symptomatic relief (P=0.003). The maximal voluntary pressures were similar to those at base line in both groups, and the resting anal pressure was reduced by 25 percent in the treated group but not in the control group. Three patients in the control group later underwent sphincterotomy, and 10 received botulinum-toxin injections (20 U). Of the latter, seven had healed fissures after two months; the other three left the study and underwent surgery. Four patients in the treated group were later re-treated (with 25 U of botulinum toxin); all had healed fissures after two months. One patient in the control group had temporary flatus incontinence after treatment with botulinum toxin. No relapses occurred during an average of 16 months of follow-up. Local infiltration of botulinum toxin into the internal anal sphincter is an effective treatment of chronic anal fissure.