We report the histologic and ultrastructural findings on two sisters with familial visceral myopathy who presented with acquired megacolon that necessitated subtotal colectomy. Both patients were mentally retarded and had repeated episodes of constipation and fecal impaction. Each presented near the age of 30 with massive dilatation of the colon and without clinical evidence of small intestinal involvement. Histologic abnormalities primarily involved smooth muscle and included marked nuclear enlargement and irregularity, interstitial fibrosis, and cytoplasmic vacuolation. These changes were most severe in the muscularis propria, but similar abnormalities were found in the muscularis mucosae and blood vessels. In the most advanced stages, collagen had completely replaced the muscularis propria, with extreme thinning of the intestinal wall. Abnormalities were noted in all segments of the colon and the appendix, but there was little correlation between severity of involvement and the segment examined. This study not only confirms the variable nature of morphologic changes in familial visceral myopathy, but also provides evidence of more extensive involvement of intestinal smooth muscle than has been previously reported.