Endosonography of peri-anal and peri-colorectal fistula and/or abscess in Crohn's disease. Transcolorectal endosonography (ES) was performed in 36 patients with Crohn's disease suspected clinically to have a fistula or abscess. A hypoechoic or anechoic duct-like lesion immediately adjacent to the anorectal lumen compatible with a fistula was found in 32 patients. A communication between the fistulous tract and adjacent structures such as the skin, anal canal, or vagina was detected in all 32 patients. An anechoic cavity adjacent to or communicating with the fistula was visualized in 29 of the 36 patients. A fistula was visualized in the remaining seven patients with no evidence of an abscess. This anechoic cavity compatible with an abscess was surgically confirmed in 14 of 17 patients. We judged the extent and configuration of the abnormalities to be more clearly visualized by ES when results were compared with fistulography in five patients. There were no ES complications, and we conclude that ES is the preferred diagnostic procedure in patients with peri-rectal pathology because of the low risk of bacterial dissemination and low incidence of patient discomfort. Utilizing ES after non-surgical treatment was successful in 19 patients for documentation of the response to therapy. 