Beta blockers and the primary prevention of nonfatal myocardial infarction in patients with high blood pressure. A population-based, case-control study was conducted to determine whether beta blockers, used for the treatment of high blood pressure, prevent first events of coronary heart disease. All study subjects were health-maintenance organization enrollees with pharmacologically treated hypertension. Patients presented in 1982 to 1984 with new coronary heart disease, and control subjects were a probability sample of eligible hypertensive enrollees free of coronary heart disease. With the investigators blind to case-control status, the subjects' medical records were reviewed for other coronary risk factors, and the health-maintenance organization's computerized pharmacy database was used to ascertain the use of beta blockers. A larger proportion of controls than cases were using beta blockers. This difference was confined to the subgroup with nonfatal myocardial infarctions. For current use, the estimated relative risk for nonfatal myocardial infarction was 0.62 (95% confidence interval, 0.39 to 0.99). Among current users of beta blockers, higher doses conferred greater protection. Past use and total lifetime intake of beta blockers were only weakly associated with case-control status. The current use of beta blockers may prevent first events of nonfatal myocardial infarction in patients with high blood pressure. 