The level of circulating endotoxin was determined in the sera from children hospitalized for treatment of diarrhea at the International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh. A significant level of endotoxin was detected in the sera from 23 (54%) of 42 children. On the other hand, in 32 normal controls, endotoxin was below the limits of detection of the assay. Antibiotic and fluid therapy markedly reduced the level of serum endotoxin and improved the general condition of most patients. Non-survivors (n = 5) had higher levels of circulating endotoxin before treatment than survivors (n = 37), suggesting a significant correlation between the serum endotoxin level before treatment and mortality. Malnutrition did not affect the serum endotoxin level in the patients with diarrhea. It was suggested that infection of Gram-negative bacteria might be involved in a significant number of patients with diarrhea in Bangladesh and that endotoxin might play a pathogenic role in those patients.