In 65 recipients of marrow from HLA-identical siblings, the death rate from graft-versus-host disease (GVHD) at 1 year was 13%, which was significantly less when compared with 50% among 10 recipients of marrow from HLA-non-identical donors (P less than 0.001 log-rank test). Among recipients of HLA-matched marrow the following was found: patients below 17 years of age had a death rate from GVHD of 4% compared with 32% in elder patients (P less than 0.01). In patients who received greater than or equal to 3 X 10(8) marrow cells/kg 3% died of GVHD compared with 31% in those receiving fewer marrow cells (P less than 0.02). Furthermore, death by GVHD was 11% among nonsplenectomized patients compared with 60% in splenectomized patients (P less than 0.001). Among all children, and among nonsplenectomized patients, those receiving less than 3 X 10(8) marrow cells/kg had a significantly increased death rate by GVHD. The number of T cells or T cell subsets in the marrow graft was not correlated with GVHD or death by GVHD.