Studies have shown that more than one third of all travelers to tropical countries experience an illness while abroad. Naivete about diseases, such as malaria, travelers' diarrhea, cholera, and HIV infections, appears to be the most crucial factor determining travel-related morbidity rates. A study was designed both to evaluate the level of disease-related, pretravel knowledge prior to a specialized travel clinic consultation as well as to correlate the level of knowledge with demographic and travel variables and previous travel experience. Using a self-administered questionnaire, data were collected from 167 clients who attended the Hospital for Tropical Diseases Travel Clinic, London, England, in June 1991. The questionnaire evaluated clients' knowledge and attitudes of travel-related diseases, their prevention and treatment, and the scores were analyzed using the Statistical Analysis System software, Version 5. Predictors for increased knowledge scores were examined by a one-way layout model comparing disease knowledge scores with variables such as sex, age, socioeconomic class, ethnic background, previous travel experience, type and duration of travel, destination, accommodation quality, and health insurance coverage. A step-down regression model was constructed to control for the effects of the many mutually confounding factors. Travelers' diarrhea and malaria risk at destination were acknowledged by more than 85% of the respondents, and 60% of the questions on travelers' diarrhea prevention, compared to 81% of those on malaria prevention, were answered correctly. Significantly, however, only one half of the respondents indicated that they would take appropriate measures concerning diarrhea while abroad. Stepwise regression identified previous travel experience, pretravel advice, traveling for work purposes, and using backpack overland tours as significant predictors of high knowledge. This study points out the necessity of pretravel education and that it should focus not only on malaria but also on other travel-related diseases like travelers' diarrhea. As well, target groups for whom more detailed information should be provided are identified, including inexperienced travelers, those using organized tours and sleeping indoors, those planning to visit friends or relatives, and those who have not received sufficient pretravel health advice.