Objective To investigate whether the systemic inflammation induced by chlamydial infections might be associated with symptoms of polycystic ovary syndrome (PCOS). Design Nested case-control study. Setting A questionnaire including questions about hirsutism and oligo-amenorrhea was distributed to a representative sample of women (at age 31) from the general population-based Northern Finland Birth Cohort. Those who reported both symptoms were defined as symptomatic (n = 81). Patient(s) A representative sample of women (at age 31) from the general population-based Northern Finland Birth Cohort. Intervention(s) None. Main Outcome Measure(s) To test the presence of serum antibodies to Chlamydia pneumoniae (IgG titers ≥32) and Chlamydia trachomatis (IgG titers ≥8) by microimmunofluorescence in symptomatic and control women. Result(s) Antibodies were investigated in 79 symptomatic and 1427 control women ( C. pneumoniae ) and in 79 symptomatic and 425 control women ( C trachomatis ). C. trachomatis antibodies (odds ratio OR, 2.4; 95% confidence interval CI, 1.3–4.6) and C. pneumoniae antibodies (OR, 1.5; 95% CI, 1.0–2.4) were more commonly present in symptomatic women, and the simultaneous presence of elevated highly sensitive C-reactive protein levels strengthened this association. Conclusion(s) Chronic inflammation, which is associated with chlamydial infections, could contribute to the pathogenetic processes that lead to the metabolic and hormonal disorders of PCOS.