In Sweden, there are large differences in the incidence of myocardial infarction (MI) among occupational groups. These differences may to some extent be due to work environment factors, including psychosocial job strain. The aim of the present study was to estimate the relative risk (RR) of MI for Swedish men and women in high strain occupations as compared to those in low strain occupations. The association between job strain and MI was studied by case-control methods. The study base comprised the population of four Swedish counties from 1976-1981 and of Stockholm county from 1976-1984. Incident cases of MI were identified through hospital discharges together with deaths. Information about occupation was obtained from the 1970 and the 1975 censuses and individuals were characterized with regard to job strain on the basis of their job title. In all, 9295 cases and 26 101 controls with unchanged type of occupation from 1970 to 1975 were included in the analyses. Men and women aged < 65 in high strain occupations showed an RR of 1.1-1.4, and men age < 55 an RR of 1.2-1.6 compared to those in low strain occupations. Similar results were obtained in analyses of male white and blue collar workers respectively. If the association between job strain and MI is causal this could be of great importance in explaining differences in MI incidence among occupational groups in Sweden.