OBJECTIVE To compare the prevalence of Chlamydia trachomatisinfections in ankylosing spondylitis (AS) patients with controls, using DNA amplification assays in urine specimens. METHODS 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. RESULTS No significant differences were found between cases and controls in the prevalence ofC 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). CONCLUSION The prevalence of Chlamydia trachomatisinfections, as detected by commercially available DNA amplification assays in urine specimens, in AS patients is not higher compared with male controls of the same age. However, there seems to be an association between specific antibodies to C trachomatis and AAU.