This is a phase II study to evaluate the effect of pembrolizumab on the duration of remission in acute myeloid leukemia. Pembrolizumab is given after complete remission is obtained in those with AML at least 60 years old who are not candidates for allogeneic stem cell transplant. The primary purpose of this study is determine if the time to relapse can be extended. Additionally, the safety and tolerability of pembrolizumab will be closely monitored.