The puborectalis muscle is a major determinant in the control of fecal continence. With endorectal sonography puborectalis function was assessed by determining puborectalis length in the resting (D1) and contracted (D2) states and calculating the percentage of shortening (D1-D2/D1)(100). The puborectalis length was defined as the shortest distance between the symphysis pubis and the rectal mucosa. The mean puborectalis length at rest did not differ in control subjects (29.5 +/- 1.0 mm) and incontinent patients (29.0 +/- 2.9 mm). With voluntary contraction a statistically significant difference (P < 0.001) was seen in the percentage of shortening between control subjects (15.4 +/- 1.1%) and incontinent patients (4.7 +/- 2.8%). Endorectal ultrasonography is a useful method to diagnose fecal incontinence.