To examine the association between the level of treated blood pressure and the incidence of myocardial infarction, we conducted a population-based case-control study of 912 members of a health maintenance organization who were receiving standard clinical treatment for hypertension. We found a J-shaped relationship between the most recently measured diastolic blood pressure and the risk of myocardial infarction, the lowest risk occurring at 84 mm Hg. The relative risk of myocardial infarction at 60 mm Hg was 2.07 (95% confidence interval, 0.86 to 5.01), and at 100 mm Hg, 1.45 (95% confidence interval, 1.02 to 2.06). Treated systolic pressure bore a linear relationship to the risk of infarction. We conclude that the optimum target range for diastolic blood pressure in hypertensive patients may be 84 to 90 mm Hg. Levels outside this range may be associated with increased risk of myocardial infarction.