The aim of the study was to explore the impact of birthplace and socio-economic characteristics on the risk of acute myocardial infarction (AMI) in a rural Swedish province. A case control study was performed and cases were all patients 30-72-y-old treated for the first time for an AMI during 1980-1992 and identified in a computerised hospital register. Three controls for each case were randomly selected from the national population register and were matched for gender, age and municipality. Birthplace ie within or outside the province, and a variety of socio-economic background factors were determined for cases and controls. The study comprised a total of 18 948 individuals including cases and controls. Altogether 4737 cases of AMI were identified whereof 3514 were men. Being born in the province was associated with the lowest risk for AMI in both sexes (RR=0.62, CI 0.57-0.67 for men and RR=0.61, CI 0.53-0.69 for women). A substantially reduced risk for men was found in farmers. In both sexes, high socio-economic status had a favourable effect on the risk to suffer an AMI. Being born in the province carried the lowest risk for AMI in both sexes and was independent of other tested socio-economic factors (income, education, occupation, and civil status). Province characteristics might be associated with this protective factor. Alternatively, migration as such might include unfavourable exposures that increase the risk for AMI among those who had moved to the province.