Frequencies of HLA A, B, C, and DR antigens were calculated in 1009 leukaemia patients, all of whom had been treated with bone-marrow transplantation and reported to the International Bone Marrow Transplant Registry. Cw3 (relative risk = 2 X 26, p = 0 X 00002) and Cw4 (relative risk = 2 X 18, p = 0 X 00003) were significantly associated with acute leukaemia (p values adjusted for multiple comparisons). Cw3 (relative risk = 2 X 97, p = 0 X 00002) and Cw4 (relative risk = 2 X 10, p = 0 X 004) were also significantly associated with acute lymphoblastic leukaemia and Cw4 was significantly associated with acute myelogenous leukaemia (relative risk = 2 X 39, p = 0 X 0003). The data suggest that Cw3 and Cw4 may be markers for leukaemia susceptibility genes, genes that code for low immune responsiveness to putative leukaemia viruses, or genes that regulate the response to chemotherapy and, therefore, the attainment of remission, since most of these patients underwent transplantation during remission.