A World Health Organization Working Group has developed a major international collaborative study with the objective of measuring over 10 years, and in many different populations, the trends in, and determinants of, cardiovascular disease. Specifically the programme focuses on trends in event rates for validated fatal and non-fatal coronary heart attacks and strokes, and on trends in cardiovascular risk factors (blood pressure, cigarette smoking and serum cholesterol) in men and women aged 25-64 in the same defined communities. By this means it is hoped both to measure changes in cardiovascular mortality and to see how far they are explained; on the one hand by changes in incidence mediated by risk factor levels; and on the other by changes in case-fatality rates, related to medical care. Population centres need to be large and numerous; to reliably establish 10-year trends in event rates within a centre 200 or more fatal events in men per year are needed, while for the collaborative study a multiplicity of internally homogeneous centres showing differing trends will provide the best test of the hypotheses. Forty-one MONICA Collaborating Centres, using a standardized protocol, are studying 118 Reporting Units (subpopulations) with a total population aged 25-64 (both sexes) of about 15 million.