Escherichia coli strains of serotype O157:H7 have been incriminated in outbreaks and sporadic cases of food-borne illness, including diarrhea, hemorrhagic colitis, hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. Food-producing animals, particularly cattle, are believed to be reservoirs of the organism. Whether all strains of bovine origin pose human health risk is unknown and was the impetus for this investigation. We compared the virulence of ten SLT-I, SLT-II, and eae DNA probe-positive O157:H7 strains from cattle to 10 like strains associated with human diarrheal disease outbreaks for virulence in one day-old gnotobiotic pigs. All strains caused diarrhea, and only four pigs inoculated with either of two bovine strains failed to develop that condition. Signs of central nervous system disease, death, debilitation requiring euthanasia before the end of an eight day observation period, and/or encephalomalacia occurred in 32/42 pigs inoculated with the strains isolated from human beings, 13/39 pigs inoculated with strains from cattle, and 7/7 pigs inoculated with a positive control strain. More strains of human origin (9/10) than bovine origin (5/10) caused these effects. The results of this study indicate considerable variability in virulence of O157:H7 strains possessing the same known virulence determinants, and suggest that disease outbreaks tend to be caused by the more virulent of these strains.