OBJECTIVE: to compare the ability of two independent Chlamydia pneumoniae antibody tests to predict need for small abdominal aortic aneurysm (AAA) repair. PATIENTS AND METHODS: annual scans were offered to 149 screening diagnosed small AAA (<5 cm). Serum samples were collected for measuring IgA and IgG-antibodies to C. pneumoniae by microimmunofluorescence (MIF) test and the new ELISA (Labsystems). RESULTS: a significant concordance was found between MIF and ELISA titres with Kappa values of 0.29 for S-IgA and 0.42 for S-IgG. IgG antibodies measured by ELISA were most predictive for cases expanding operation recommendable sizes with a sensitivity and specificity of 80% and 66%, respectively. CONCLUSION: the simpler EIA has a high correlation with the MIF test and both were predictive for the natural history of AAA. Chlamydia antibody test may be used to identify individuals who might benefit from follow-up and anti-chlamydia treatment. Copyright 2001 Harcourt Publishers Limited.