Allogeneic hematopoietic stem cell transplantation (HSCT) has allowed patients to be cured from previously incurable cancers or hematopoietic diseases by ablating the host immune system and infusing healthy blood stem cells from a healthy donor. Graft failure, the absence of cellular recovery after HSCT, is a significant complication of transplant. When graft failure is diagnosed late, as it frequently is, the outcome is devastating. We have identified novel imaging and blood biomarkers that can detect subclinical engraftment early after HSCT and could expedite this diagnosis and rescue through re-transplantation. In our published study, the imaging biomarker, (18)F-fluorothymidine (FLT) PET/CT, detected subclinical engraftment quantitatively in adults within 5 days of HSCT, illuminating the pathway of subclinical cellular repopulation in bone marrow. All patients engrafted and there were no toxicities associated with the procedure. Our study also showed that the serum enzyme, thymidine kinase 1 (TK1), a novel blood biomarker of HSC proliferation, increased co-incident with cellular recovery. Collectively, these data suggest that FLT imaging and TK1 blood levels may have the potential to distinguish between subclinical engraftment and graft failure. We now propose to evaluate whether these biomarkers can identify delayed engraftment or failure in the patients who are at highest risk for this complication: recipients of cord blood and haplo-identical HSCT. We hypothesize that FLT uptake will identify subclinical engraftment early after HSCT in alternative donor transplant settings and that FLT and TK1 will serve as predictive biomarkers of graft failure. We will test these in three specific aims: 1) To determine whether FLT marrow signal intensity identifies subclinical engraftment and maps distribution of early marrow settling after cord blood or haplo-identical transplantation, 2) To determine whether FLT marrow signal intensity distinguishes between engraftment and graft failure in very high-risk alternative donor HSCT recipients, and 3) To determine whether serum TK1 enzyme levels can distinguish subclinical engraftment from graft failure. Upon completion of these aims, we expect to show that these blood and imaging biomarkers can predict graft failure in patients at highest risk for this complication. If confirmed, FLT and TK1 could be used to identify primary graft failure early after HSCT, permitting expeditious rescue HSCT and significantly improved outcomes.