The daily number of cases of myocardial infarctions admitted to a hospital in middle Sweden over three winter seasons 1984-87 was correlated to the weather conditions on a day-to-day basis. The study encompassed 634 days and all cases younger than 70 years, living within the catchment area, in all 382 subjects. Information on temperature, wind force, precipitation and atmospheric pressure was obtained from the Swedish Institute of Meteorology and Hydrology. A low number of myocardial infarctions was seen on Saturdays and Sundays with a mild wind chill factor and on days with moderate snowfall and high atmospheric pressure. A high number was observed for workdays, especially Mondays, as day of diagnosis. Heterogeneity of the study population and a misclassification of the time relationships between dates of diagnosis and weather changes may have caused an underestimation of the impact of weather conditions. However, weather conditions do not seem to be a major triggering factor of myocardial infarctions in Sweden.