The WHO MONICA Project is designed to measure the trends in mortality and morbidity from coronary heart disease (CHD) and stroke, and to assess the extent to which they are related to changes in known risk factors in different populations in 27 countries. Risk-factor data are collected from population samples examined in at least two population surveys (one at the beginning of the study and the other at the end). The results of the baseline population surveys are presented. In populations studied, the proportion of smokers varied between 34-62% among men and 3-52% among women. The population median of systolic blood pressure varied between 121-146 mmHg in men. In women the figures were 118 mmHg and 141 mmHg respectively. In diastolic blood pressure, the variation of median was from 74 mmHg to over 91 mmHg among men and from 72-89 mmHg among women. The third major risk factor considered was total cholesterol, with the population median ranging between 4.1-6.4 mmol/l among men and 4.2-6.3 mmol/l among women. Caution is required when making cross-sectional comparisons between the risk-factor levels as the MONICA Project was not designed for this purpose. Nevertheless, these data demonstrate clearly the large variety of baseline risk-factor patterns in populations studied in the MONICA Project.