As the economic integration of Mexico and the United States intensifies, so does the cross-migration of labor forces. Subsequently, when migrant workers or their families become ill, health care is often disjointed and suboptimal. Binational health data exchange among providers of health care becomes essential. GUAPA (incorporating the first three letters of Guanajuato and the state abbreviation for Pennsylvania) is a demonstration. project in Guanajuato (Mexico) and Pennsylvania that represents a vital step toward the assessment of health risk, epidemiological surveillance, and assurance of quality health care for mobile populations. The pilot system is designed to test the technical, financial, legal, and political aspects of data sharing between the two countries. Demographic and clinical data on Mexican migrant patients are collected and made available to front-line health care providers in Mexico and the United States. Data transfer concentrates on four tracer conditions of binational concern: sexually transmitted diseases, tuberculosis, leprosy, and hepatitis. This chapter presents details on the development of the data transfer system, including system objectives, system design, software and data entry procedures, data analysis, procedures to protect client confidentiality, and preliminary evaluation. While GUAPA demonstrates that the exchange of health information is technologically possible with relative ease and minor expense, a larger challenge is becoming clear--to surmount political, cultural, and economic barriers that impede provision of health care on both sides of the border. (Author/SV)