Escherichia coli isolates (n=2629) were collected between 1996 and 2000 from 2100 Thai children less than 12 years of age with acute diarrhoea. Enterotoxigenic (ETEC), enteroinvasive (EIEC), Shiga-toxin-producing (STEC), enteropathogenic (EPEC) and enteroaggregative (EAEC) E. coli were identified by their virulence marker profiles, as determined by multiplex PCR, and HeLa cell-adherence patterns. Serogroups of isolates were determined using 43 monovalent O antisera. Of 2629 isolates, 16.9% were identified as diarrhoeagenic E. coli, and the mean isolation rates per year were 10.2% for EAEC (range 8-12.5%), 3.2% for EPEC (0-8%), 3.0% for ETEC (2-5.4%), 0.5% for EIEC (0-1%) and 0.04 % for STEC (0-0.1%). The isolation rates of pathotypes from four different age groups (0-5 months, 6-11 months, 1-2 years and 2-12 years) in 905 children whose ages were recorded were respectively 19.3, 18.2, 9.1 and 8.1% for EAEC, 3.1, 4.3, 1.7 and 2.2% for EPEC and 2.6, 2.3, 1.3 and 5% for ETEC. About 38% of diarrhoeagenic E. coli, including 55.1, 66.7, 100, 45.9 and 29%, respectively, of ETEC, EIEC, STEC, EPEC and EAEC, and 24% of non-diarrhoeagenic E. coli were O-antigen typable. Only four serogroups (9.3%) were restricted to single pathotypes, whereas 27 serogroups (62.8%) were not restricted to any pathotype. This study shows that EAEC are the most prevalent diarrhoea-associated pathotype in Thai children.