PURPOSE: This study was undertaken to analyze whether intra-anal ultrasound examination, anorectal physiologic evaluation, and histopathologic examination in patients with chronic idiopathic anal pain presented any common features and whether the results of different treatment modalities correlated with these findings. METHODS: Eighteen patients who met the criteria for chronic idiopathic anal pain were studied. All had an intra-anal ultrasound examination and a complete anorectal physiologic evaluation. In a selected group of patients, ultrasound-guided biopsy samples were taken from pathological areas in the internal and external sphincter. Treatment consisted of analgesics only in four patients, 0.2 percent nitroglycerin ointment in four, and ultrasound injection of botulin (botulinum toxin, Botox®) into the intersphincteric space in nine. Two patients, including one who was previously treated with botulin, ultimately had a colostomy. RESULTS: Four patients were managed satisfactorily on analgesic treatment under the guidance of the hospital's pain clinic. Nitroglycerin ointment resulted in temporary pain relief in one of four patients. Injection of botulin resulted in a permanent improvement in four patients, a temporary improvement in one patient, and no effect in four patients. Two patients had a colostomy, resulting in complete pain relief. The effect or lack of effect of nitroglycerin ointment and botulin was not related to changes in anal pressure. CONCLUSION: Chronic idiopathic anal pain is a condition of unknown origin for which no proven therapy exists. As in other syndromes based on muscular dystonia, some patients may benefit from injection of botulin.