SUMMARY In an epidemiological survey in Troms0, northern Norway a prevalence of definite ankylosing spondylitis (AS) of between 1-1% and 1-4% was found (males: 1 9-2 2% and females: 0-3-0.6%). The ratio of male to female was between 3 9 and 6-1 in favour of the male sex. It was calculated that 6-7% of the B27 positive individuals had AS, and that 22-5% of the B27 positive subjects with back pain or stiffness suffered from AS.