Although the risk for acute myocardial infarction (AMI) is lower after transplantation than on the waiting list, this risk may vary by patient population and may be different early versus late after transplantation. Risk factors for AMI were examined among 53,297 Medicare beneficiaries who were placed on the deceased-donor waiting list in 1995 to 2002. Early (<or=3 mo) and late (>3 mo) effects of receiving a deceased- or living-donor kidney transplant were examined using time-dependent covariates in Cox nonproportional hazards analysis. Overall, transplantation was associated with a 17% lower adjusted risk for AMI (0.83; 95% confidence interval [CI] 0.77 to 0.90) versus the waiting list. However, the relative risk (versus the waiting list) for AMI was greater for deceased- compared to living-donor transplants, with both being much greater early (deceased-donor 3.57 [95% CI 3.21 to 3.96] compared to living-donor 2.81 [95% CI 2.31 to 3.42]) than late (deceased-donor 0.45 [95% CI 0.41 to 0.50] compared to living-donor 0.39 [95% CI 0.33 to 0.47]) posttransplantation. Individuals who were >or=65 yr of age had a much higher risk (versus 18- to 34-yr-olds) for AMI early posttransplantation (8.01; 95% CI 5.12 to 12.53) compared with the waiting list (3.68; 95% CI 3.98 to 4.54) or late posttransplantation (4.37; 95% CI 3.07 to 6.20). Black patients had less reduction in AMI risk (versus white patients) late posttransplantation (0.78; 95% CI 0.64 to 0.95) compared with early posttransplantation (0.60; 95% CI 0.48 to 0.74) or on the waiting list (0.62; 95% CI 0.56 to 0.68). The AMI risk that was associated with chronic kidney disease from diabetes (versus glomerulonephritis) was relatively greater on the waiting list (1.64; 95% CI 1.45 to 1.85) compared with early (1.34; 95% CI 1.08 to 1.68) and late (1.39; 95% CI 1.12 to 1.72) posttransplantation. Thus the risk reduction for AMI with transplantation versus the waiting list varies by patient population and time after transplantation.