Background: To determine whether there is an ecological association between antibiotic use and chlamydia prevalence. Methods: A systematic review was undertaken of international studies on chlamydia prevalence among women aged 15-25 years published between 2000 and 2005. Preference was given to studies using nucleic acid testing and representative population-based sampling methods. Data were obtained on per capita antibiotic consumption according to the defined daily dose. Results: For the 12 countries for which both antibiotic consumption and relevant prevalence data for chlamydia were available, a non-significant negative correlation was found between total antibiotic consumption per capita and chlamydia prevalence among younger women according to country (rs = -0.242, P = 0.449). When an outlier (from the Netherlands) was excluded, the correlation was significant (rs = -0.615, P = 0.044). Combined use of tetracyclines and macrolides was also associated with lower chlamydia prevalence (rs = -0.697, P = 0.017). Conclusions: It is possible that antibiotics used for other reasons may have unexpectedly reduced the prevalence of chlamydia.