The objective of this prospective study was to determine anal sphincter function and thickness of the anal musculature in patients with myotonic muscular dystrophy. Manometric studies were performed in 16 patients with myotonic dystrophy and in 16 healthy controls. Patients had significantly lower basal and squeeze pressures than control subjects (P less than 0.01). The results of ultrasonographic studies of the anal canal in 7 patients and 7 control subjects suggest that this decrease in muscle strength is partly explained by muscular atrophy. In addition, patients with myotonic dystrophy showed exaggerated rebound contractions following and sphincter relaxation that was induced by rectal distention. The pattern of this response and the results of electromyographic studies in 6 patients with myotonic dystrophy suggest that such abnormalities are explained by a neurogenic defect rather than a myotonic response of the anal musculature. It is concluded that patients with myotonic dystrophy show a multitude of defects in the anal sphincter that are an expression of myopathy, muscular atrophy, and neural abnormalities.