The MONICA Project was undertaken by the World Health Organization (WHO) as a ten year international effort to measure the trends in cardiovascular mortality and coronary heart disease and cerebrovascular disease morbidity and to assess the extent to which these trends were related to changes in known risk factors, daily living habits, or major socioeconomic features. Pilot studies were completed in 1981 in four countries and by 1985 there were 41 MONICA's in defined communities in 27 countries. Each study site covered a population of around 300,000 persons and was expected to be studied for a decade. The centers measured the incidence of cardiovascular disease--particularly coronary heart disease and cerebrovascular diseases--and assessed the extent to which these trends were related to changes in known risk factors (smoking, blood lipids, blood pressure, diet), daily living habits, health care, and major socioeconomic factors in the study populations. MONICA was also expected to introduce, in the countries concerned, new health statistical methods that would lead to more effective use of national health information systems.