PURPOSE: The fatigue rate index (FRI) has been developed to access sustained voluntary contraction of the external anal sphincter. This study is designed to refine the technical aspects of measuring the FRI and to re-evaluate its clinical significance. METHODS: Thirty-eight patients with fecal incontinence (19 males, 19 females) and 37 (21 males, 16 females) controls were studied. Anal manometry was performed by using standard protocols. Fatigue was measured over a 20-sec squeeze at a high-pressure zone. The FRI was calculated from the maximum squeeze pressure (MXSP) and the fatigue rate. For the accuracy of the calculation, the mean resting pressure (MRP) was calibrated to zero before the metric procedure, representative segments of the calculation were selectively designated as those denoting sustained squeeze, and representative channels were designated from among 8 channels by using 3 or more of those showing typical sustaining-pressure curves. RESULTS: No difference in demographic factors was detected between the two groups. Basic manometric parameters were significantly different between the two groups (MRP, 29.4+/-21.3 vs. 44.4+/-17.8 mmHg, P=0.0015; MXSP, 205.9+/-98.0 vs. 274.2+/-106.5 mmHg, P=0.0051). The FRI was also significantly different (29.8+/-14.3 vs. 86.3+/-127.1 sec, P=0.0108). The areas under the receptor operating-characteristic curves for the MRP, the MXSP, and the FRI were 0.72, 0.69, and 0.84, respectively. The sensitivity and the specificity of the FRI for detecting fecal incontinence were 80% and 65% at 40 sec as a cut-off point. CONCLUSION: The FRI was proven to be more accurate than the MRP and the MXSP in detecting incontinence. With adequate modifications of the measuring method, measurement of the FRI could be done more easily and conveniently, but its cut-off point for detecting fecal incontinence was lower than previously reported.