In Finland, the incidence of coronary heart disease is reported to be about twice as high as in the neighbouring country of Sweden. The aim of this study was to find out what happens to this excess risk among Finnish immigrants to Sweden. A geographically defined population in Stockholm County was followed up in the in-patient care registry and the cause-of-death registry in order to find cases for a case-control study. All episodes of myocardial infarction (ICD numbers 410.00 and 410.99) that had occurred in men born between 1911 and 1935 during the period 1974-76 were recorded. An age-matched control group was randomly selected from the population. By means of the civil registration system it was possible to identify country of birth and length of stay in Sweden. Of the 356 cases, 32 (9%) had been born in Finland compared with 55 (6%) of the 887 controls. The overall relative risk for the Finnish immigrants compared with native Swedes was 1.7. For the group of Finnish immigrants who had been in Sweden for 20 years or more the relative risk was 1.3. It was concluded that since the relative risk decreases during the stay in Sweden the environmental factors dominate the explanation of the difference in the relative risk of developing myocardial infarction between Finland and Sweden.