Passive transmission of autoimmune diseases by allogeneic stem cell transplantation is rare and is ascribed to passive transfer of memory B-cells from donor to recipient. We hereby report a case of transmission of an asymptomatic lupus anticoagulant from a sibling donor to a recipient of transplantation for secondary acute myeloid leukemia. On pre-harvest evaluation, the sibling donor with no history of bleeding or thrombosis was found to have a lupus anticoagulant. After engraftment, the recipient was found to have a new prolonged activated partial thromboplastin time and was subsequently shown to have a lupus anticoagulant on Day +73 after stem cell transplantation. The recipient remained well with no evidence of bleeding, thrombosis, or graft-versus-host disease and was on a stable dose of tacrolimus at the time the lupus anticoagulant was detected. There was no other identifiable trigger for the appearance of a lupus anticoagulant.