Bone marrow transplantation (BMT) is the treatment of choice in children affected by primary immunodeficiency (PID). Because only 10-15% of affected children have a familial HLA-identical donor alternative therapeutic options are BMT from a matched unrelated donor or an haploidentical BMT. In our experience only 40% of these children find a donor within the International Registry. Therefore, the remaining 50% children affected by PID are candidates for haploidentical BMT. Unfortunately, in PID other than sever-combined immunodeficiency (SCID), low engraftment rates have been reported because of minimal residual immunity. In order to enhance engraftment rate in haploidentical BMT in PID we suggest a protocol with addition of donor peripheral stem cells after mobilization with granulocyte colony-stimulating factor (G-CSF) (16 micrograms/kg for 5 days) and bone marrow cells. This procedure increases the cell load, which allows intensification of the conditioning regimen for induction of faster engraftment. The separation of CD34+ cells from leukapheresis products was achieved in the first 6 patients by the Isolex 300 system (Baxter) with a CD34+ cell purity range of 80-95% and in another three patients by the Clinimacs System (Miltenyi). The peripheral blood stem cells were cryopreserved until BMT, 15 days after G-CSF stimulation when the bone marrow was harvested, processed and T-cell depleted with Campath 1-M in the first 6 cases while the Clinimacs System was used in the remaining cases and no T-cell depletion was required. We included 9 patients in the study protocol: SCID (4), Omenn's syndrome (3), LAD (1) and CID (1). The mean value of peripheral CD34+ cells infused was 13.42 x 10(6)/kg and the mean CD3+ cells number was 0.385 x 10(5)/kg; the mean value of BM CD34+ cells infused was 10.62 x 10(6)/kg and the mean CD3+ cell number was 2.39 x 10(5)/kg. The mean number of infused CFU was 8.1 x 10(5)/kg for PBSC and 3.59 x 10(5)/kg for BM. The 9 patients achieved more than 0.5 x 10(9) peripheral blood neutrophils/L at a mean of 14.6 days (range: 6-22 days). One patient affected by SCID showed complete chimerism, but he died after BMT of systemic CMV infection; the other 8 patients are alive and well and 4 of them show complete chimerism in all cell lines. Split chimerism was documented in 2 SCID cases (CD3+ lymphocytes were of donor origin, monocytes were autologous and granulocytes were mainly autologous); 1 patient affected by Omenn's syndrome received 3 transplants (1 from the mother and 2 from the father, T-cells alone and bone marrow) and achieved engraftment with complete chimerism after the third transplant; the patient affected by LAD also received 3 transplants (2 bone marrow infusions and 1 PBSC infusion) achieving complete chimerism after the third one. In conclusion, the engraftment achieved in all treated patients, and the acceptable conditioning-related toxicity suggest that this approach could be successfully applied to children affected by PID and candidates for haploidentical BMT.