BACKGROUND: Although long-term beta-blocker therapy has been found beneficial in patients after an acute myocardial infarction, these drugs are greatly underused by clinicians. Moreover, the dosages of beta-blockers used in randomized controlled trials appear to be much larger than those routinely prescribed. OBJECTIVE: To determine whether an association exists between the dosage of beta-blockers prescribed after a myocardial infarction and cardiac mortality. METHODS: We performed a retrospective cohort study of 1165 patients who survived an acute myocardial infarction from January 1, 1990, through December 31, 1992. These patients represent a subgroup of the 6851 patients hospitalized at northern California Kaiser Permanente hospitals. RESULTS: Of the 37.7% of patients prescribed beta-blocker therapy, 48.1% were treated with dosages less than 50% of the dosage found to be effective in preventing cardiac death in large randomized clinical trials (lower-dosage therapy). Compared with patients not receiving beta-blockers, those treated with lower-dosage therapy appeared to have a greater reduction in cardiovascular mortality (hazard ratio, 0.33; P=.009) than patients treated with a higher dosage (hazard ratio, 0.82; P=0.51), after adjustment for age, sex, race, disease severity, and comorbidities. CONCLUSIONS: The dosages of beta-blockers shown to be effective in randomized trials are not commonly used in clinical practice, and treatment with lower dosages of beta-blockers was associated with at least as great a reduction in mortality as treatment with higher dosages. This suggests that physicians who are reluctant to prescribe beta-blockers because of the relatively large dosages used in the large prospective clinical trials should be encouraged to prescribe smaller dosages.