Background and aims: Dividing or "laying open" of the tract for intersphincteral type anal fistula is simple and effective and entails low risk of complication, but little is known of the functional and manometric the clinical and manometric effect of this surgery on anal sphincter function. Patients and methods: The study examined 45 adults undergoing surgical treatment for intersphincteral fistula. We administered the questionnaire for continence score and performed anorectal manometry before the operation and at least 6 months after the operation. The operative method was laying open of the fistula tract and trimming the redundant anoderm for adequate drainage of the wound in all cases. Results: There was a significant decrease in maximal resting anal pressure and in resting pressure throughout the distal 2 cm of the anal canal after operation. The maximal contractile pressure that before operation. Continence control was significantly poorer in women and patients who had lower preoperative resting pressure. Multivariate analysis showed lower preoperative resting pressure to be the only independent factor for impaired continence control after fistula surgery. Conclusion: Although laying open of the fistula tract is a simple and effective therapy for intersphincteral type anal fistula, it should be more conservative for patients with low resting anal pressure.