Erythrocyte chimerism, graft versus host reaction, and course of disease were studied in patients subjected to bone marrow transplantation from 40 HLA identical sibs and 7 monozygotic twins. Disease relapses were observed in 35% of patients after allogenic and much more often in those after isogenic bone marrow transplantation. Relapses occurred in all patients subjected to transplantation from monozygotic twins. These results may be explained by the genetic and biological deficiencies of bone marrow cells from HLA identical sibs. 15% of recipients developed an acute graft versus host reaction after transplantation from HLA identical sibs; this is three times less than after transplantation from genetically unrelated donors, as reported elsewhere.