CD34+ cell selection of PBPC after harvest from G-CSF-treated allogeneic donors results in a more than 200-fold depletion of T lymphocytes in the graft and has been used to reduce the incidence of acute GVHD post transplant. Since transplantation with T cell-depleted BM grafts is associated with a delay in immune reconstitution and an increase of opportunistic infections, we evaluated the immunological reconstitution of patients with hematologic malignancies after therapy followed by CD34+-selected PBPC34 transplantation from matched related donors. Lymphocyte subset reconstitution over the first 12 months post transplant and the incidence of infections were evaluated in 12 patients receiving PBPC34 grafts and compared to that of patients after transplantation with PBPC without CD34+ enrichment (n = 20) or unmanipulated bone marrow grafts (BM; n = 15). PBPC34 grafts contained 264-fold fewer T lymphocytes (median 0.53 x 10(6) kg/body weight) than PBPC grafts and 36-fold fewer than BM grafts (140 x 10(6)/kg and 19 x 10(6)/kg, respectively). Despite a two log depletion of T cells in the PBPC34 grafts, T lymphocyte reconstitution appeared comparable among the three transplant groups over the first 12 months. A positive patient CMV serostatus pretransplant was correlated with a faster T cell reconstitution in all transplant groups. GVHD prophylaxis with methylprednisolone delayed B lymphocyte reconstitution. The incidence of infections post transplant did not appear to be increased in the PBPC34 group compared with the PBPC and BMT groups. It remains to be shown in larger prospective trials, whether these promising preliminary data of lymphocyte reconstitution and the clinical course after transplantation with PBPC34 can be confirmed.