BK virus is well known to be associated with allograft failure in renal transplant recipients due to polyomavirus-associated nephropathy (PVAN). BK viruses are increasingly recognized an important cause of hemorrhagic cystitis during hematopoietic stem cell transplantation (HSCT). Here we describe two patients with acute lymphoblastic leukemia who received a haplo identical match related donor hematopoietic stem cell transplant after myeloablative conditioning and post-transplant cyclophosphamide. They developed cytology proven BK Virus cystitis. Both were treated with Leflunomide. Treatment was effective in alleviating the symptoms temporarily but eventually the symptoms progressed and resulted in mortality of both patients.