In acute myocardial infarction, intravenous beta blocker therapy has been tested in about 30 controlled, randomized trials. Of these, the ISIS-1 using atenolol and the MIAMI trial using metoprolol are the most important. In a total of 26,437 patients, total deaths were reduced by 62 during day 1 and by 64 during the first week, i.e. 97% of the lives were saved during the first day of beta blocker treatment. In post-myocardial infarction, oral beta blocker maintenance treatment has been used in about 35,000 survivors in placebo-controlled trials. Of these, the timolol, metoprolol and propranolol (BHAT) trials are the most important. In the timolol trial lasting for 33 months, total death, total cardiac death and re-infarction rate were significantly reduced. In the metoprolol study lasting for 3 months, total and cardiac mortality were reduced, and in the BHAT study lasting for 25 months fatal and non-fatal re-infarction combined was significantly reduced. Primary prevention of coronary heart disease has been the intention in hypertension trials. Despite the fact that beta blockers are potent agents against elevated blood pressure, a well-established coronary risk factor, no controlled trial with a placebo or untreated control group has shown a definite preventive effect on coronary heart disease. The reason for this apparent paradox is not known, but many speculations have been aired that the lack of effect might be due to adverse metabolic effects of most beta blockers.