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Despite its proven efficacy, β-blocker therapy remains underused in elderly patients after myocardial infarction (MI). The objectives of this study were to identify undertreated groups of seniors and to determine whether older and frailer patients are being selectively dispensed low-dose β-blocker therapy.From a comprehensive hospital discharge database, all people aged 66 years or more in Ontario who survived an acute MI between April 1993 and March 1995 were identified and classified into those who did not receive β-blocker therapy and those dispensed low, standard or high doses of this agent. Logistic regression models were used to study the effect of age, sex, comorbidity, potential contraindications to β-blocker therapy and residence in a long-term-care facility on the odds of not being dispensed a β-blocker. Among β-blocker users, the odds of being dispensed low relative to standard or high doses of this agent were evaluated.Of the 15 542 patients, 7549 (48.6%) were not dispensed a β-blocker. Patients 85 years of age or more were at greater risk of not receiving β-blocker therapy (adjusted odds ratio [OR] 2.8, 95% confidence interval [CI] 2.5-3.2) than were those 66 to 74 years. Having a Charlson comorbidity index of 3 or greater was associated with an increased risk of not receiving β-blocker therapy (adjusted OR 1.5, 95% CI 1.3-1.8) compared with having lower comorbidity scores. Patients who resided in a long-term-care facility were at increased risk of not being prescribed β-blocker therapy (adjusted OR 2.6, 95% CI 2.0-3.4). Among the 5453 patients with no identifiable contraindication to β-blocker therapy, women were significantly less likely than men to receive this agent (p = 0.005). Of the 6074 patients who received β-blockers, 2248 (37.0%) were dispensed low-dose therapy. Patients aged 85 years or more had an increased risk of being dispensed low-dose therapy (adjusted OR 1.6, 95% CI 1.3-2.0) compared with those aged 66 to 74 years. Compared with those who had the lowest comorbidity scores, patients with the highest comorbidity scores were more likely to be dispensed low-dose β-blocker therapy (adjusted OR 1.3, 95% CI 1.0-1.8).Almost half of Ontario patients aged 66 or more who survived an MI, particularly those who were older or frailer, did not receive β-blocker therapy. Among those dispensed β-blocker therapy, older and frailer patients were more frequently dispensed low-dose therapy.