Trans-rectal ultrasound with a 5 MHz probe was used to examine 16 patients who had perianal fistulas and 5 of them underwent one or more surgical explorations. In 11 cases, all surgically checked, sonography was able to demonstrate a pararectal abscess and accurately image the site, size, extension and--in some cases--the fistula itself. It can be advised that, using high frequency probes (7 or 10 MHz), transrectal ultrasound could provide even more exact diagnostic information in anorectal pathology.