We determined costs associated with diarrhea in a <36-month-old ambulatory population. Children with acute diarrhea were enrolled during the rotavirus season at three centers. Questionnaires to assess costs of both medical and nonmedical factors were administered at the enrollment visit and 1 week later. Office computer records were reviewed to identify all visits by children with diarrhea during 1 year. Fifty-one patients were enrolled. The average cost per episode of diarrhea was †289, which included: †144, missed work; †57, office visits; †23, laboratory tests; †21, medications; †18, changed diet/oral rehydration solutions; †15, travel; †7, extra diapers; and †6, extra child care. During 1 year diarrhea accounted for 4% of all visits and 10% of visits among those <36 months old. The annual cost at the three centers was †346 000, which extrapolates to †0.6 to †1.0 billion for the United States. Twenty-one percent of this cost was attributable to rotavirus diarrhea. We conclude that outpatient care for pediatric diarrhea is a major health care cost in the United States.