GeoSentinel is a network of 22 member travel/tropical medicine clinics (14 in the United States and 8 in other countries) initiated in 1995 by the International Society of Travel Medicine (ISTM). GeoSentinel is based on the concept that these clinics are ideally situated to effectively detect geographic and temporal trends in morbidity among travelers. The core surveillance tool is a single-page faxable form submitted to a central data site for each post-travel patient, including immigrants, refugees, and foreign visitors. Diagnoses are entered either as specific etiologies or as syndromes and are then linked to geographic locations, reference dates, and clinical presentations. In addition, electronic communication with the larger body of worldwide ISTM member clinics is periodically done to obtain broader data collection in response to specific inquiries. The scope of GeoSentinel has broadened from the initial vision of a provider-based sentinel network tracking emerging infections at their point of entry into developed countries. Its present goals are (1) to monitor global trends in disease occurrence among travelers; (2) to ascertain risk factors and morbidity in groups of travelers categorized by travel purpose and type of traveler; (3) to respond to urgent public health queries; (4) to develop educational priorities for travelers' health; and (5) to effect a rapid response by electronically disseminating alerts to surveillance sites, to all ISTM members in 55 countries, and to public health authorities. In addition, a major byproduct of the network, and now one of its strongest assets, has been the growth of partnerships between ISTM, Centers for Disease Control and Prevention and health-care providers around the world, as well as other medical societies, government, and private organizations. The demographic data, travel patterns, and clinical presentations for the first 2813 patient records analyzed from the GeoSentinel sites are summarized in this paper.