 OUTLINE: Beginning on day 14 after allogeneic hematopoietic stem cell transplantation HSCT, patients undergo blood sample collection at least once weekly for the first 100 days and then once to twice monthly for up to 1 year, in the absence of graft-versus-host disease GVHD. If chronic or acute GVHD develops after day 100, more frequent blood sampling may occur. Blood sample collection is coordinated with the time of regular clinic visits to allow for evaluation of the clinical events recorded 2 weeks before and after the blood draw date. Blood samples are analyzed by the Cylex and ImmuKnow assays to measure global T-cell immune function and responsiveness to alterations in immunosuppressive post-HSCT therapy. Assay data obtained during the first 3 weeks and other post-transplant periods will be evaluated for possible correlations with clinical endpoints i.e., GVHD incidence, rate of infection, and response to immunosuppressive therapy to assess the predictive value of the assay. 