Studies were made in Göteborg over a period of 6 years of all cases of acute myocardial infarction diagnosed among men below the age of 40. Thirty-six cases were registered and 8 of these died outside hospital. Three patients died early during the hospital stay. The remaining 25 patients were compared with a random sample from the general population in Göteborg with respect to conventional risk factors. Smoking, and high plasma cholesterol values were dominating findings among the patients, whereas there was no significant differences in blood pressure levels. Coronary angiography was performed in 18 patients of whom 2 showed normal coronary arteries and left ventricular angiograms. These 2 patients were the only ones free from risk factors. Of the remaining 16 patients, 10 had only one vessel affected.