We studied 2,246 episodes of childhood diarrhea over a 2-year period in a general hospital serving a population of about one million on the island of Hong Kong. Rotavirus (24%) and nontyphoid salmonellas (23%) were the most common causal agents, followed by Campylobacter sp. (9%). Rotavirus occurred largely during winter, whereas salmonellas and Campylobacter sp. occurred more commonly during summer and autumn, respectively. Enterotoxigenic Escherichia coli was rarely isolated (1%), and the isolates were clonally diverse. A small percentage (2.8%) of E. coli had serological specificities commonly associated with enteropathogenic E. coli, but only two of the isolates were also positive for HEp-2 adhesive factor. Pure or heavy and predominant growth of E. coli was obtained in repeated stool cultures of 432 (19%) of these episodes, which did not yield any of the above-mentioned pathogens. Although associated with diarrhea, these E. coli isolates possess neither the pathogenic attributes of enterotoxigenic E. coli nor the properties commonly associated with enteropathogenic E. coli. Enteroinvasive E. coli and enterohemorrhagic E. coli were considered unlikely causes on clinical grounds.