Diarrhea and unexpected death were encountered in a group of young Syrian hamsters (Mesocricetus auratus) used for hyperlipoproteinemia and atherosclerosis research. The animals were fed an atherogenic diet containing 18% saturated fat and 0.366% cholesterol. Mortality began 45 days after hamsters were placed on this atherogenic diet. The atherogenic studies were aborted at 74 days because of high mortality. Toxigenic Clostridium difficile was isolated from animals found dead or euthanatized because of illness. Signs observed were unexpected death and acute liquid diarrhea. Characteristic pathologic changes were necrosis and hemorrhage of the intestinal mucosa with acute inflammation. Hepatic lipidosis was a consistent finding presumed to be associated with the consumption of the atherogenic diet. The study was repeated by placing 23 hamsters on the atherogenic diet and 10 hamsters on the control diet. In animals fed the atherogenic diet, the average time to mortality differed between studies, but clinical signs, gross and histologic lesions, culture findings, and toxin results in both atherogenic diet groups were similar. C. difficile was not isolated from the feeds. No antibiotics were found in the atherogenic diet. The results from these studies suggest that hamsters fed an atherogenic diet have increased susceptibility to disease caused by C. difficile as compared with hamsters fed a normal fat and cholesterol diet.