The 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-27 positive son.