Anal endosonography offers detailed images of the sphincter muscles, the rectal wall and their surroundings. An axial endoprobe with a large image sector, preferably 360 degrees, and high frequency (7-10 MHz) is required for assessment of fistulous tracts. Endosonography can give information about the relation of the fistula to the sphincter muscles and the levator plate, the internal opening and possible abscesses. Endosonography is equal or better than MRI with an external coil. MRI with an endocoil can give more detailed images than endosonography, but the endosonographic examination is cheaper and faster. Using endosonography during injection of hydrogen peroxide through the external opening apparently improves the accuracy. Abscesses are easily depicted by endosonography, and guided puncture can be performed using the transrectal, transanal or transperineal route, depending on the location of the abscess. Repeated punctures may be necessary. Alternatively, a drainage catheter can be placed using the transrectal route.