The optimal approach to stem cell transplantation in children with immunodeficiency who lack a matched family donor is controversial. Unrelated donor stem cell transplantation gives equivalent outcome to mismatched family donor stem cell transplantation in severe combined immunodeficiency, whereas unrelated donors may be preferable in non-severe combined immunodeficiency children. However, unrelated donor stem cell transplantation with conventional conditioning regimens has been associated with significant treatment-related toxicity, particularly in non-severe combined immunodeficiency patients with preexisting organ dysfunction. We report the outcome of a series of 33 consecutive unrelated donor transplantations performed at our center in children with primary immunodeficiency using a reduced-intensity conditioning regimen between 1998 and 2001. We have compared these outcomes with a retrospective control cohort of 19 patients who underwent transplantation with myeloablative conditioning between 1994 and 1998. All children in both groups had primary engraftment. There was no statistical difference in the speed of immune reconstitution or incidence of graft-versus-host disease between the 2 groups. Overall survival was significantly better in the reduced-intensity conditioning group: 31 (94%) of 33 patients survived, compared with 10 (53%) of 19 in the myeloablative conditioning group (P = .014). We conclude that the reduced-intensity conditioning regimen results in improved survival and reduced transplantation-related mortality compared with myeloablative conditioning in high-risk patients undergoing unrelated donor transplantation.