The incidence and etiology of travelers' diarrhea was studied in a crew of 1,914 sailors and marines aboard a U.S. Navy ship during a western Pacific deployment. Questionnaires completed by 301 troops indicated that 52% had at least one episode of diarrhea during the deployment; however, only 5% of the ship's company sought treatment. Enterotoxigenic Escherichia coli was the most commonly identified pathogen (23%), followed by Giardia lamblia (6%), Salmonella (3%), rotavirus (2%), and Shigella, Campylobacter jejuni, and Entamoeba histolytica (1% each). In 66% of the episodes no etiologic agent was found. None of the risk factors thought to be associated with travelers' diarrhea could be statistically associated with the diarrhea group in comparison to questionnaire respondents who denied having had the illness.