OBJECTIVES: To analyse differences in myocardial infarction incidence between two Swedish counties and to evaluate the importance of major risk factors for the observed differences. DESIGN: The incidence of first myocardial infarction was studied using information from registers. For a number of risk factors of myocardial infarction, the prevalence as well as the relative risk was estimated from population controls of case-control studies in the two areas. SUBJECTS: Men and women aged 45-64 years in Stockholm and Västernorrland County 1993-94. MAIN OUTCOME MEASURES: Relative risks (RRs) and impact fractions were used to evaluate the importance of differences in risk factor prevalence for differences in myocardial infarction incidence between the two areas. RESULTS: The incidence of first myocardial infarction was higher in Västernorrland than in Stockholm amongst both men (RR = 1.23; 95% CI = 1.08-1.40) and women (RR = 1.41; 95% CI = 1.11-1.79). Obesity and increased levels of blood serum lipids were more prevalent in Västernorrland than in Stockholm amongst men with impact fractions of 6 and 9-11%, respectively. Amongst women, corresponding differences were not seen, but job strain and shift work tended to be more common in the more northern area. Current smoking was more frequent in Stockholm, particularly for women. CONCLUSIONS: The incidence of first myocardial infarction was higher in Västernorrland than in Stockholm in both genders. A higher prevalence of obesity and elevated blood serum lipids may explain, in part, this excess incidence amongst men, but amongst women the causes of the higher incidence in the more northern area remain largely unclear.