['To compare the prevalence of Chlamydia trachomatis infections in ankylosing spondylitis (AS) patients with controls, using DNA amplification assays in urine specimens.', 'The prevalence of C trachomatis infections was assessed in 32 male AS patients and 120 age and sex matched controls. Urine specimens were tested by ligase chain reaction and polymerase chain reaction. In addition, blood samples of AS patients were tested on serum antibodies to C trachomatis (IgA and IgG) by a specific peptide based solid phase enzyme immunoassay. A questionnaire was used to assess the differences in sexual behaviour and ethnic origin between the two groups. AS patients were also asked about disease characteristics.', 'No significant differences were found between cases and controls in the prevalence of C trachomatis infections. No associations were found between C trachomatis antibodies and disease characteristics, except for acute anterior uveitis (AAU). Four of eight (50%) AS men positive for IgG had a history of AAU in comparison with three of 24 (12.5%) IgG negative men (OR = 7.0; 95% confidence intervals: 1.1, 44.1).'] Is prevalence of Chlamydia trachomatis in urine of male patients with ankylosing spondylitis increased?