SUMMARYHLA‐A,B,C and DR antigen frequencies were determined in a group of 188 patients suffering from acute myeloid (AML) and acute lymphoid leukaemia (ALL). These antigen frequencies were compared with those obtained on a panel of normal individuals (n= 109) of the same ethnic origin. The significance of the differences in the antigen distribution and the strength of the associations between particular HLA antigens and the disease were then calculated. The resuks obtained show a decreased frequency of HLA‐Awl9 in the overall group of patients and the group of patients with ALL. In addition, the antigen frequency of the HLA‐B18 and DRS(DRw11) antigens was also decreased in the overall group of patients and in those patients with AML but not in the patients with ALL. The results suggest that the antigen Aw l9 may confer some degree of resistance to the development of ALL and that the HLA‐B18 and/or DR5 antigens may be resistance factors for the development