The Cronkhite-Canada syndrome is a rare non-hereditary disorder with generalised gastrointestinal polyposis, associated with ectodermal changes. We report here a case of adenocarcinoma and multiple adenomas of the large intestine associated with Cronkhite-Canada syndrome in a 61-year-old Japanese man. Histologically, the rectal tumour was composed of well-differentiated tubular adenocarcinoma, admixed with foci of adenomatous components, and associated with hyperplastic mucosa of Cronkhite-Canada syndrome. Multiple polyps, >20 polyps < or = 2.0 cm in diameter, were found near the carcinoma of the resected rectum. Histologically, superficial parts of the polyps were composed of tubular adenomas, and basal parts of the polyps were hyperplastic dilated glands. It was speculated that, in the present case, the rectal adenocarcinoma arose from mucosal hyperplasia (Cronkhite-Canada polyp)-adenoma-carcinoma pathway. This suggested that Cronkhite-Canada syndrome has definite malignant potential, although the frequency of malignant transformation is thought to be low in Cronkhite-Canada syndrome.