To identify Escherichia colithat cause infantile diarrhea in Bangkok, Thailand, E. coli isolated in a year-long study of infantile diarrhea were examined for 0 and H serotypes and virulence determinants. Classic enteropathogenic E. coli (EPEC) were isolated from 28 of 509 infants with diarrhea (cases) and 11 of 509 age-matched controls (P = .009; odds ratio [OR], 2.64). Most of this difference was attributable to EPEC adherence factor (EAF)-positive EPEC that produced an attachment and effacement lesion, as identified in the fluorescence actin staining assay, isolated from 13 cases and 1 control (P = .003; OR, 13.3). EAF- EPEC was isolated from 15 cases and 10 controls (P = .418; OR, 1.52) and EAF+ non-EPEC from 17 cases and 10 controls (P = .242; OR, 1.72). EAF+ EPEC that caused an attachment and effacement lesion was found in 3% of children <6 months old with diarrhea who were studied in an outpatient clinic in Bangkok in 1988.