This is a Phase II open label study of crenolanib besylate. This study will enroll subjects with relapsed or refractory AML with FLT3 activating mutations. Prior treatment with other FLT3 TKIs is allowed. Subjects will take crenolanib 200mg/m2/day divided in three doses daily (preferably every eight hours), taken orally at least 30 minutes pre or post meal until disease progression, death, or the patient discontinues treatment for adverse events, investigator's judgment, or other reasons. Patients who are able to proceed to allogeneic stem cell transplant will be able to resume crenolanib therapy post-transplant in an attempt to maintain remission.