SummaryThe authors have provided a new recurrence risk table of ankylosing spondylitis. The mathematical problem solved was to combine a continuous distribution for polygenic inheritance with a discrete one, HLA‐B‐27 antigen ‘yes’ or ‘no’. Previously the highest probability of recurrence was 0.0785 in the case of an affected mother's son and it increased to 0.1416 when the B‐27 antigen positive affected mother had a B‐