Aberrant crypt foci (ACF), identified as putative precursor lesions in the development of colon cancer, were induced by brominated trihalomethanes (THMs) administered in the drinking water of rats. To investigate whether ACF induced by THMs could be promoted by a diet high in saturated animal fat, male F344/N rats were exposed to 0.5, 0.7, 0.9 or 1.1 g/l of trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and tribromomethane (TBM), respectively, in drinking water. All animals were fed a Purina 5001 diet with half receiving the normal 4.5% fat feed and half receiving feed supplemented with 19% animal fat. After 26 weeks of treatment, colons were excised and examined for ACF. No difference in ACF was noted between animals fed a normal or high fat diet and exposed to TCM, BDCM or DBCM. However, animals exposed to TBM and fed a high fat diet showed a significant and near two-fold increase in ACF when compared to TBM exposed animals fed a normal diet.