'Carcinoma of the colon does not occur in cases of megacolon' is an axiom held by Brazilian physicians working in endemic areas for Chagas' disease. The objective of the present study was to test this axiom experimentally by submitting rats with experimental megacolon to a carcinogen which causes carcinoma of the colon. Eighty young male Wistar rats received serosal application of either saline (0.9% NaCl) or 2 mM benzalkonium chloride (BAC) to the distal colon. Ten months later randomly chosen saline and BAC rats were injected weekly with dimethylhydrazine (DMH) for 20 weeks. Non-DMH-treated rats from both original groups were maintained, for a total of four experimental groups. Three months after the injections all surviving rats were killed. At autopsy the presence of absence of carcinomas along the colon was recorded. The induction of megacolon was evaluated by morphometry of the wall from the distal colon and myenteric denervation was assessed by neuron counts. An increase of at least 2-fold in distal colon wall thickness confirmed the induction of megacolon in BAC-treated rats. Neuronal counts from BAC and control rats not treated with DMH showed an average denervation of 63%. The number of distal colon carcinomas in BAC+DMH-treated rats was significantly lower than that in DMH-treated rats. These findings appear to contradict the traditional concept of carcinogenesis of the colon. The clinical axiom was reproduced experimentally.