The nature and incidence of life-threatening or fatal toxicity after marrow transplantation were analyzed in 52 patients who received HLA-A, B, DR, Dw-phenotypically identical marrow transplants from unrelated volunteer donors (URD). An age and disease-matched cohort of 104 patients transplanted from HLA-genotypically identical siblings was used as a comparative group. The actuarial probability of grade 3 or 4 regimen-related toxicity was 31% after URD transplants and 21% after matched sibling transplants (log-rank p = 0.1041). The median duration of first hospitalization was 33 days for recipients of genotypically-identical marrow and 36 days for recipients of URD marrow (p = 0.0244). Sixty percent of genotypically identical marrow recipients and 51% of unrelated volunteer donor marrow recipients were discharged home from Seattle free of disease at a median of 99 days post-transplant (p = 0.5095). The percentage of patients requiring readmission to hospital and the actuarial probability of requiring either hemodialysis or mechanical ventilation were not statistically different between the two groups. We conclude that matched URD marrow transplantation is not associated with significantly greater regimen-related toxicity or mortality than transplantation from HLA-genotypical siblings.