T-cell depletion of the marrow graft using counterflow centrifugal elutriation reduces the risk of graft-versus-host disease (GVHD). However, because of high rates of graft failure and relapse, elutriation alone has not improved survival. We have carried out a phase II clinical trial in 54 pediatric patients to determine if CD34+ selection to rescue pluripotent stem cells from the small lymphocyte fraction improves engraftment. The processed grafts contained a mean of 5.5 x 10(7) cells/kg IBW, 4.7 x 10(6) CD34+ cells/kg IBW, and 6.3 x 10(5) CD3+cells/kg IBW. Patients achieved an ANC >500 at a median of 16 days and platelet count >20 000 at a median of 28 days. The incidence of clinically significant GVHD was 19%. In total, 10 patients enrolled in this study experienced graft failure, with eight of the 14 patients transplanted for nonmalignant indications failing to engraft stably. Graft failure was statistically significantly associated with nonmalignant diagnosis (P<0.001), but was not associated with CMV seropositivity, donor gender, or cell counts of the allograft. We conclude that although time to engraftment is similar to that seen with unmanipulated grafts, graft failure remains a significant problem in patients with hereditary, nonmalignant diseases. Future efforts will seek to preserve the benefits of elutriation with CD34+ selection by increasing immune ablation of the preparative regimen and/or increasing posttransplant immune suppression.