The diagnosis of congenital megacolon can only rarely be established in adult cases with dilatation of the rectum and sigma, since the narrow aganglionic segment and increased levels of plasma acetylcholinesterase activity are usually lacking in adult cases,--these findings being typical for congenital megacolon. Idiopathic megacolon thus is the correct diagnosis in most of these adult cases. Surgical removal of the dilated parts of the intestine and end-to-end anastomosis yield satisfactory results, thus a colostomy is not necessary.