The role of common diarrheal pathogens in dehydration was examined in children with acute watery diarrhea who attended the treatment center of the International Centre for Diarrhoeal Disease Research, Bangladesh, in Dhaka. Two hundred sixty-nine children with moderate or severe dehydration were matched with 700 children with no dehydration. Vibrio cholerae O1 infections were 5.5 times more likely to be associated with dehydration than in cases without this agent. No significant association could be found between the presence of enterotoxigenic Escherichia coli, Campylobacter jejuni, or rotavirus infection and dehydration. These results were obtained after simultaneously controlling for age, lack of oral rehydration therapy (ORT) at home, protein energy malnutrition, withdrawal of breast-feeding during diarrhea at home, poor housing, longer duration of diarrhea at home, and delay in reaching the treatment center. The cholera isolation rate was only 4.5% and thus explains only a small proportion of the cases of dehydration. In cholera-endemic areas, a strategy to prevent dehydration in small children is needed to ensure correct use of ORT at home, prompt referral, and the use of a suitable antibiotic when cholera is clinically suspected.