Anal endosonography is a new technique that is useful in the preoperative assessment of patients with anal fistulas. Endosonographic images are created by the reflection of sound waves from the interfaces between tissues of varying densities. In order to accentuate tissue interface layers at the level of the fistula tract, we introduced hydrogen peroxide into the fistula tract through the external opening during anal ultrasonography in two patients with recurrent anal fistula. Hydrogen peroxide injection resulted in hyperechoic imaging of the preinjection hypoechoic horseshoe fistula tract. Endosonographic findings were confirmed at the time of surgery in both patients. We conclude that hydrogen peroxide enhancement of the fistula tract is a simple, effective, and safe method of improving the accuracy of endoanal ultrasound assessment of recurrent anal fistula.