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Objectives: To assess the effectiveness of β blockers in short term treatment for acute myocardial infarction and in longer term secondary prevention; to examine predictive factors that may influence outcome and therefore choice of drug; and to examine the clinical importance of the results in the light of current treatment.

Design:Systematic review of randomised controlled trials.

Setting: Randomised controlled trials.

Subjects: Patients with acute or past myocardial infarction.

Intervention: βBlockers compared with control.

Main:outcome measures All cause mortality and non-fatal reinfarction.

Results: Overall, 5477 of 54 234 patients (10.1%) randomised to β blockers or control died. We identified a 23% reduction in the odds of death in long term trials (95% confidence interval 15% to 31%), but only a 4% reduction in the odds of death in short term trials (−8% to 15%). Meta regression in long term trials did not identify a …