A patient is described who had unexplained, vague abdominal pain for 7 years and subsequently presented with worsening pain and mild dilatation of the distal small intestine. Gross examination of a resected ileal segment demonstrated thinning of the muscularis propria. Light- and electron-microscopic examination revealed a myopathy which preferentially involved the external longitudinal layer of the muscularis propria and was characterized by vacuolar degeneration of smooth muscle cells. No inflammatory infiltrate or neural or vascular lesions were present. These features characterize hollow visceral myopathy and differentiate it from other myopathic and degenerative disorders which involve gastrointestinal smooth muscle.