Considerable regional variation in coronary heart disease mortality has been observed in Sweden. This variation is of public health concern and is also a point of departure for etiologic studies. The central aim of the present study was to investigate whether there are also regional differences in myocardial infarction incidence and to evaluate whether these are likely to be explained by regional differences in smoking, blood pressure, and serum cholesterol. The study base consisted of subjects age 30-64 living in eight Swedish counties during 1976-1981. Cases of myocardial infarction were identified using both hospital discharge and death records. A 30-40% increased incidence of myocardial infarction was found in the two northernmost counties compared with Stockholm county. The regional differences in myocardial infarction incidence were not related to differences in smoking but might be due in part to differences in serum cholesterol and blood pressure, and to socioeconomic factors as well.