Of 35 patients with acute myocardial infarction (AMI) at the age of 40 years or less, 32 (91%) smoked and only three patients were non-smokers. The age at AMI related significantly to the extent of smoking (p < 0.001, Kruskall-Wallis test). Five patients with AMI at the age < 30 years smoked more heavily than the 30 with AMI at the age of 30-40 years (p = 0.04, Mann Whitney U test). Heavy smoking men > 30 years at the AMI had a Q-wave infarction as often (11 of 13 (85%)) as those with multivessel disease or a coronary artery occlusion (8 of 9 (89%) and 14 of 16 (88%) respectively) on coronary arteriography after the infarction. Smoking may be the most important modifiable risk factor in young patients with AMI.