In 1985, the US Peace Corps developed a computerized epidemiological surveillance system to monitor health trends in over 5500 Peace Corps Volunteers working in development projects in 62 countries worldwide. Data on 31 health conditions and events are collected monthly from each country; quarterly and annual incidence rates are then calculated, and the analysed data are distributed. In 1987, the most commonly reported health problems were diarrhoea (unclassified), 48 cases per 100 volunteers per year; amoebiasis, 24 per 100 volunteers per year; injuries, 20 per 100 volunteers per year; bacterial skin infections, 19 per 100 volunteers per year; and giardiasis 17 per 100 volunteers per year. Tracking each of these common problems, as well as other selected health conditions, guides design of more specific studies and disease control efforts. Health problems with very low rates (less than 1.0/100 volunteers/year) include hepatitis, schistosomiasis, non-falciparum malaria, and filariasis. The epidemiological surveillance system provides the health data needed to plan, implement, and evaluate health programmes for Peace Corps Volunteers, and provides a model for surveillance in other groups of temporary and permanent residents of developing countries.