• We studied 15 cases of severe protracted diarrhea due to a strain of Escherichia coli serotype 0111:K58:H2. The clinical features of these patients were compared with those of 18 patients infected with other enteropathogenic serotypes of E coli. More patients infected with 0111:K58:H2 strains had been treated with ampicillin (six of 15 v one of 18), and more were dehydrated (nine of 15 v two of 18) and in a toxic condition (nine of 15 v three of 18). The number of stools per day (14.1 v 6.8), the total duration of the diarrhea (24.7 v 7.0 days), and the frequency of relapses (six of 15 v none of 18) were also greater in these patients. A large number had a serum sodium level below 135 mEq/L (ten of 15 v none of 18) and a serum protein level below 11 mEq/L (six of 15 v none of 18). At least two infants Infected with E coli 0111:K58:H2 manifested a urinary tract infection. All 15 strains of E coli 0111:K58:H2 studied adhered to HeLa cell lines, whereas none of the strains Isolated from the second group were adherent. None of the 0111:K58:H2 strains produced enterotoxins, and they were negative in the Serény test. All adherent strains were resistant to many antibiotics.