From 1978 to 1985, 2,465 male bus drivers in the three major cities in Denmark were followed with regard to hospital admission due to myocardial infarction (MI) and death due to ischemic heart disease (IHD). In all 2,045 (83%) of these men responded in 1978 to a questionnaire on psychosocial well-being and work conditions. The respondents did not differ from the nonrespondents regarding hospital admissions and death in the follow-up period (1978-1984). Sixty-two cases of MI were registered among the 2,045 bus drivers in 1978-1984. On this basis relative risk for MI was calculated with a multiple regression model for independent variables regarding psychosocial well-being and work conditions. High work load (driving in heavy traffic) was significantly associated with the occurrence of MI. Of the psychosocial factors "no social contact with colleagues" and "increased work pace" were also significantly associated with the occurrence of MI. Smoking habits tended to be associated with the occurrence of MI, while stress symptoms and job dissatisfaction did not. The mental burden on bus drivers working in heavy traffic seems a possible explanation for the findings.