The functional outcome of fistula surgery can be quantitated by anal manometry. A closed, water-filled microballoon (0.5×1.0 cm) system was used to measure resting anal pressure and maximal squeeze pressure in 47 patients with anal fistulas at St. Mark's Hospital. After treatment of intersphincteric fistulas, there was a significant reduction in resting pressure in the distal 2 cm. In treated transphincteric fistulas and suprasphincteric fistulas, anal pressure was reduced in the distal 3 cm. A significant lower pressure was measured in patients having the external sphincter divided, compared with those having the muscle preserved. Disturbance of continence was related to abnormally low resting pressure in six patients. This study supports attempts at sphincter preservation in fistula surgery.