It is estimated that five million Americans will travel to the developing world over the next year. This study examines the demographic profile, past medical and immunization history, itinerary, and reason for travel of 2, 445 travelers to the developing world seen at a travel medicine service from 1984 through 1989. The travelers age ranged from three months to 85 years (mean age 43). A chronic medical condition was reported by 654 (27%). Four percent of all travelers were intolerant of sulfonamides, and 9% had contraindications to mefloquine for malaria prophylaxis. Many travelers were due to receive the primary series or updatings of routinely recommended immunizations: 43% for tetanus/diphtheria, 55% of those born after 1956 for measles, and 70% for polio if their travel itinerary included a polio risk. Most travel (71%) was for vacations, 13% was for teaching or study, 11% for business, and 5% for missionary activities. The median duration of travel was 21 days; 5% traveled for more than one year. While over 150 countries were visited, 52% of all travel was to 10 countries in East Africa, the Indian subcontinent, the Far East, and South America. Information about the epidemiology of travel to the developing world can help physicians and travel medicine services develop more effective preventive measures for travelers.