In amyotrophic lateral sclerosis the striated pelvic floor sphincter muscles are functionally uninvolved, and pathological studies have confirmed the relative resistance of the Onuf nucleus motor neurons. We have evaluated the external anal sphincter (EAS) muscle in 16 patients with ALS using single fiber EMG, and compared the results with the findings in the semimembranosus-semitendinosus (SM-ST) muscles that have innervation from the L-5, S-1, and S-2 segments. The results were compared with a group of controls matched for age and sex. None of the patients or controls had symptomatic sphincter involvement and none of the 4 women studied were parous. Eight patients with ALS showed an increased fiber density in the EAS; 6 had an abnormal neuromuscular jitter. In 1 there was fibrillation in the EAS. In the SM-ST muscle 11 patients showed an increased fiber density, and 7 had an abnormal neuromuscular jitter. In 3 patients with ALS in whom there were abnormal findings in the EAS the bulbocavernosus reflex and pudendal nerve evoked potentials were normal. Neurogenic change was more marked in the SM-ST than in the EAS muscle. These findings show that the EAS is not normal in ALS. However, the relative resistance of the EAS to ALS is sufficient to prevent incontinence, even in the longer-surviving older patients.