Diarrhea is by far the most frequent health problem of travelers to developing countries. Of the estimated 300 million international travelers who will cross the world's frontiers this year, at least 16 million persons from industrialized countries, including more than 8 million US residents, will travel to developing countries. Approximately one third of these travelers to developing countries will get diarrhea. The economic impact of travelers' diarrhea (TD) is substantial, because fear of sickness is one of the major deterrents to tourism. International tourists worldwide spend over $100 billion annually, and the economies of many nations depend on this travel. For educational, recreational, political, and financial reasons, this international exchange should be fostered. The incidence of TD varies markedly by destination and may depend in part on the number of dietary indiscretions made by the traveler and on the style of travel. Travelers' diarrhea is caused by a variety of infectious agents, and the spectrum of clinical illness varies considerably. However, this illness in travelers is usually not severe; high fever, vomiting, or bloody stools occur only in a minority of cases. Dietary prudence and hygienic measures are safe and simple preventive techniques, but they do not eliminate entirely the risk of diarrhea. Prophylactic measures such as antidiarrheal drugs and oral antimicrobial agents have been used. Travelers' diarrhea is treated with a variety of regimens, including oral electrolyte solutions, antidiarrheal compounds, and antimicrobial drugs, prescribed either singly or in combination. There continues to be a debate concerning whether the risk of antimicrobial agents is worth the benefit; whether early treatment of ill travelers is preferable to daily prophylaxis of all travelers; whether all currently employed treatment strategies are useful; and whether given groups of travelers, such as vacationers, students, or business travelers, should be selectively advised to follow special regimens. To resolve some of these questions, the National Institutes of Health Office of Medical Applications of Research and the National Institute of Allergy and Infectious Diseases convened a Consensus Development Conference on Travelers' Diarrhea from Jan 28 to 30, 1985. After 1 1/2 days of expert presentation of the available data, a consensus panel of epidemiologists, biostatisticians, microbiologists, pediatricians, internists, infectious diseases specialists, travel experts, and lay representatives considered the evidence and agreed on answers to the following questions. What is the epidemiology of travelers' diarrhea, and why is it important? What causes travelers' diarrhea? What prevention measures are effective for travelers' diarrhea? What treatment measures are effective for travelers' diarrhea? What should be the direction of future research?