The Swiss National Research Program on Prevention of Cardiovascular Diseases is an ongoing study of factor modification in two intervention cities compared with two control areas. The study design and the methods used for baseline assessment are described. Health check was offered to spontaneous participants and to a predetermined sample of 7950 people; those who did not participate spontaneously were personally invited. The global participation rate in the sample was 59% (69% of Swiss and 48% of foreigners). Risk factor exposure was compared between spontaneous participants and the people from the sample who came during the same period. There is no significant difference between these two populations with regard to either plasma cholesterol, blood pressure or body mass index. On the other hand, the proportion of smokers is higher among the invited people than among the spontaneous participants. These results justify the use of different populations for a proper analysis of exposure to the various risk factors.