Grades III/IV acute GVHD among HLA-A, -B serologic, and -DRB1 allele-matched pairs by number of class I mismatched loci. The incidence of grades III/IV acute GVHD was analyzed as a function of the number of class I mismatches detected by high-resolution HLA typing in HLA-A, -B low-resolution, and -DRB1 high-resolution matched donor-recipient pairs. The data presented are adjusted for competing risk factors using the proportional hazards model, rather than univariate analysis. One or more additional mismatches led to more frequent GVHD (P = .001).