The presence or absence of a double pre-beta lipoprotein (pre-beta 1+ or pre-beta 1-) peak on agarose gel electrophoresis was recorded in: a) 77 survivors of myocardial infarction, b) their first-degree relatives, and c) 148 controls. Thirty-nine percent of myocardial infarction survivors and 24% of controls had pre-beta 1+ (P less than 0.05). The segregation of pre-beta 1+ in the families of myocardial infarction survivors was consistent with autosomal dominant inheritance. In the control group, fasting serum cholesterol and triglyceride values were higher in the pre beta 1+ subjects, but the differences were not significant. We conclude that the presence of double pre-beta lipoprotein peak on electrophoresis is associated with an increased risk of myocardial infarction.