Peripheral blood progenitor cells (PBPC) represent an ideal source of stem cells for autologous transplantation because of technical advantages and more favourable engraftment kinetics. The reconstituion of a functional immune system occurs earlier in patients transplanted with cytokine-mobilized autologous PBPC compared with bone marrow; because of the greater T-cell content in PBPC products, donor-derived antigen-specific T-cells transferred with the graft might contribute to short-term immunity in transplant recipients. Despite a prompt reconstitution of B- and T-cell numbers, both B- and T-cell function are profoundly impaired for a prolonged period of time after PBPC infusion. The positive selection of CD34+ cells might provide effective tumor cell purging without compromising hematopoietic recovery in patients with acute leukemia, multiple myeloma, breast cancer and non-Hodgkin's lymphoma, whose autografts have been reported to contain malignant cells which might promote disease relapse. However, the incidence of viral infections in the early posttransplant period might be increased after CD34-selected compared with unmanipulated PBPC transplants, as a result of the lack of accessory and immune cells in the graft. The purpose of this review is to provide an update on immunological reconstitution after transplantation of autologous PBPC; in particular, emphasis will be placed on the mechanisms of immune dysfunction after the infusion of unmanipulated and CD34-selected autografts.