The use of volunteer, unrelated donors has substantially increased the number of potential donors for pediatric marrow transplantation during the past few years. We describe our single institution experience of short-term toxicity after pediatric marrow transplantation using sibling or unrelated donors. Fully matched (A, B and DR loci) donors were employed in 94% of the cases in both groups. Conditioning of similar intensity and uniform supportive care were employed in the two groups. Both primary non-engraftment and secondary graft failure were more common among recipients of unmanipulated URD grafts. Clinically significant (grades III-IV) acute GVHD and toxic mortality during the immediate post-transplant period were also higher in this group of patients. Pediatric marrow transplantation using volunteer, unrelated donors appears to be associated with an increased incidence of procedure-related toxic complications.