When analyzing risk factors for first acute myocardial infarction in the Copenhagen City Heart Study, a large prospective population study of 20,000 men and women, smoking was found to influence risk significantly in a dose-dependent manner, the risk increasing 2% to 3% for each gram of tobacco smoked daily. Risk was particularly associated with inhalation, the risk for inhalers being almost twice that of noninhalers. No difference in risk could be demonstrated between various types of tobacco (pipe, cigar/cheroots, or plain and filtered cigarettes). The risk seemed associated with current smoking only, inasmuch as the duration of the smoking habit was not important. Ex-smokers had the same risk as those who had never smoked regardless of duration of smoking and time elapsed since quitting. Relative excess risk was significantly higher in female smokers than in male smokers, and daily alcohol intake appeared to have some protective effect on the risk of first acute myocardial infarction among heavy smokers.