The effect of HLA-DR matching between living kidney donors and related recipients on 1-year graft survival was examined in 35 transplant recipients excluding 2-haplotype identical siblings. All 11 DR-compatible grafts survived 1 year; 7 of 24 DR-incompatible grafts ceased to function within 1 year (p less than 0.05). Among 24 DR-incompatible recipients, 6 of 11 who had preoperatively been transfused with less than 1000 ml blood lost their graft function. On the other hand, 12 of 13 who had received at least 1200 ml blood maintained their graft function beyond 1 year (p less than 0.05). All DR-compatible recipients received pretransplant blood transfusions and all produced anti-DR antibodies against random panel B cells after transplantation, but not against the graft donor's B lymphocytes. The anti-DR antibodies disappeared within 3 months. In recipients whose sera broadly reacted to random panel B cells at 1 years after transplantation, only 4 of 10 grafts survived. In recipients whose sera had lost their anti-DR reactivity within 1 year, all 20 grafts survived (p less than 0.05).