There has been an impression that diarrhea occurs commonly in seriously ill patients treated in ICUs. In view of the sparsity of published work on the problem, we embarked on a prospective study of all patients admitted to the ICU for more than 48 h over a 12-month period. Three factors were examined in detail: nasogastric feeding, cimetidine administration, and antibiotic treatment. Other factors also were considered, notably the nature of the underlying illness and the spread of a possible infective agent by cross-infection. There was a 41% incidence of diarrhea. A significant increase in the incidence of diarrhea occurred in patients on nasogastric feeding (p less than 0.01) and in those receiving cimetidine (p less than 0.05); there was no increased incidence in those receiving antibiotic therapy. The cytotoxin of Clostridium difficile was specifically looked for in all patients with diarrhea, but was not detected.