Less than 40% of the patients who could benefit from marrow transplantation have an HLA-matched relative who can serve as a donor. For this reason, several centers have explored marrow transplantation from other categories of donors. This retrospective study analyzes the results of marrow transplantation for 52 patients receiving grafts from HLA-A,B,DR,Dw-phenotypically matched, MLC-compatible, unrelated volunteer donors compared to a disease, disease-stage, and age-matched cohort of 104 patients transplanted from HLA-genotypically identical sibling donors. The patients transplanted from unrelated donors had an increased incidence of grade II-IV acute graft-versus-host disease compared to patients transplanted from related donors (79% vs. 36%, P much less than 0.001). However, the probability of relapse-free survival appears similar in the two groups (P = 0.39 over all, with estimates of 41% vs. 46% at 1 year). We conclude from this preliminary data that marrow transplantation from HLA-matched unrelated donors should be considered in most, if not all, circumstances where transplantation from an HLA-matched sibling would be indicated if such a donor were available.