The clinical, radiological and laboratory data of 50 HL-A B27-positive ankylosing spondylitis (AS) patients were compared with those of 12 HL-A B27-negative AS patients. The only statistically significant difference (P>0.003) was the association of acute anterior uveitis (AAU) with HL-A B27-positive patients. The possibility that AS and AAU are two discrete entities that are independently related to HL-A B27 is discussed.