The results of a prospective study of participants and non-participants in population surveys conducted in Moscow and Kaunas are presented. The average follow-up period was 5.5 years. There were 6656 male participants (P) and 5200 non-participants (NP) in Moscow and 5945 and 2528, respectively, in Kaunas. In Kaunas, indicators of both total mortality and mortality from cardiovascular diseases, injuries and accidents, malignant neoplasma and other diseases were higher in NP than in P. In Moscow the general tendency was the same but the difference between P and NP was significant only for mortality from injuries and accidents. As for the incidence of new cases of acute myocardial infarction, there was a tendency to higher figures in participants, especially in Moscow. The authors point to the danger of drawing incorrect conclusions on the mortality and morbidity in the population if they are based only on data gained from participants.