Anorectal lesions are common and can develop silently in patients with Crohn's disease. Transrectal ultrasound examinations were performed to study 40 healthy individuals and 40 patients with Crohn's disease. A rigid linear endorectal probe was used to examine the rectal wall, the perirectal tissues, and the anal sphincter. In healthy individuals, the rectal wall showed five layers with a total diameter of maximum 4 mm. The anal sphincter was clearly visualized as an echo-poor and sharply delineated structure. No pathological lesions were detected perirectally. In Crohn's disease, an enlargement of the rectal wall was seen in 16 patients and heterogeneity of the anal sphincter in 19 patients. This technique detected lesions missed with the routine proctological examinations: four pararectal abscesses, two pararectal fistulas, two para-anal abscesses, and one para-anal fistula. In all examined subjects, the anal sphincter increased in breadth during squeezing and in length during straining. It is concluded that transrectal ultrasonography sharply delineates the rectal wall and the anal sphincter at rest and under dynamic conditions and detects unknown abscesses and fistulas in the pararectal and para-anal tissues in patients with Crohn's disease.