Human leucocyte antigen (HLA)-B27 carriers are predisposed to inflammatory and autoimmune diseases. Among 1137 patients with haematological diseases, 59 HLA-B27 carriers were identified. Compared with 18 774 volunteers, the incidence of HLA-B27 was increased in patients with acute leukaemia (relative risk RR = 1·67, P = 0·002), for both acute myeloid leukaemia (AML) (RR) = 1·67, P = 0·007) and acute lymphoblastic leukaemia (ALL) (RR = 1·68, P = 0·094). Of all the HLA-B27 carriers, four patients had ankylosing spondylitis (AS), all with lymphoid malignancies (three ALL, one Hodgkin's disease), whereas no HLA-B27 carriers with myeloid leukaemia had AS symptoms (P = 0·006). This suggests that HLA-B27 carriers may have an increased risk of acute leukaemia and those with concomitant AS may be predisposed to lymphoid malignancies.