The bias introduced by non-participation in a study depends on the size and the composition of the non-participant group. Out of 10,000 men invited to a screening examination in a large primary prevention trial in Göteborg, Sweden, 25% did not come to the examination. The non-participants could be shown to be registered by the Board of Social Welfare for social problems and alcohol abuse to a greater extent than the participants in the study. The annual mortality among the non-participants was about twice that of the participants during the entire follow-up period of 11.8 years. The incidence of non-fatal myocardial infarction was not significantly higher in non-participants. Coronary death, on the other hand, was significantly more common among those not attending the examination (3.5 vs. 7.6%). Participants registered for alcoholic problems had coronary death rates approaching those of the non-participants. Sudden coronary death accounted for most of the excess coronary mortality. Most of the excess mortality in the non-participant group was from other causes than cardiovascular diseases and cancer but even so, about one third of the excess deaths could be attributed to coronary heart disease. Possible explanations of this excess coronary mortality include that non-participants may smoke more, more frequently have alcoholic problems and that non-participation may reflect an unwillingness to seek medical care even in the event of illness.