The broad, long-term objectives and specific aims are to increase progression free and overall survival following allogeneic transplantation for malignant diseases. The primary limitation to treating patients with high dose therapy followed by autologous stem cell transplantation is post-transplant relapse. The central hypothesis of this project is that allogeneic stem cells infused after a non-myeloablative regimen will induce a graft-versus-leukemia/lymphoma (GVL) response without significant graft-versus-host disease (GVHD). Two complementary strategies will be pursued in order to test this hypothesis. First, patients with leukemias and lymphomas will be transplanted with human leukocyte antigen (HLA) matched sibling donor peripheral blood stem cells (PBSC). This strategy will be followed by planned donor leukocyte infusions (DLI) given at specified times. These studies will evaluate toxicity, engraftment, GVHD, progression free and overall survival. The second approach will rely on in vitro measurements. These in vitro studies will define the kinetics of immune reconstitution in transplant recipients. These measurements will consist of general, antigen specific, alloreactive, and tumor antigen specific immune responses to define the dynamics of immune reconstitution following allogeneic transplantation in this patient population. These studies will also focus on how T-cells recover (central thymic vs. peripheral mechanisms). Moreover, these studies will determine whether GVL recognition has occurred. Specifically, autologous tumor cells or autologous tumor derived RNA transfected dendritic cells will be used as targets. The goal of this aim will be to detect donor-derived cells that can recognize these tumor targets and lyse the target cells. If a GVL effect can be detected, then the last aim will be to develop a dendritic cell vaccine targeting WT1 for allogeneic transplants. The overall goal of these two strategies (clinical and laboratory) is define the benefit of allogeneic stem cell transplantation for patients with hemato-lymphoid diseases. The lessons learned from these trials will set the stage for future studies aimed at decreasing relapse and improving long-term survival.