The rate of isolation of Escherichia coli belonging to the traditional serotypes enteropathogenic for infants was studied prospectively in two groups. Group 1 consisted of children with diarrhea and of controls without gastrointestinal disease who were matched for age and inpatient or outpatient status. Group 2 consisted of families entered in a prospective study of rotavirus infections. In group 1 enteropathogenic Escherichia coli were found in 13 (6%) of 220 children younger than 12 months of age and in nine (6%) of 143 children 12--35 months of age, all of whom had diarrhea. Enteropathogenic E. coli were found in only one of an equal number of matched controls (P = 0.002 and 0.004, respectively). In group 2 enteropathogenic E. coli were present in seven (18%) of 38 specimens obtained during diarrheal episodes, as compared with five (1%) of 492 specimens obtained when there was no diarrhea (P less than 0.001). The enteropathogenic E. coli isolated were not enterotoxigenic. The most common serogroup was O111, but many different O:H serotypes were detected. Thus, the association of enteropathogenic E. coli with endemic diarrhea was significant, even though no enteropathogenic mechanism was apparent.