BACKGROUND: To estimate the population-based chlamydia prevalence among women aged 18 to 35 years living in Melbourne, Victoria, and to assess the feasibility of using mailed urine specimens to test women. METHODS: A simple random sample of 11,001 households in Melbourne was selected from the telephone directory. Participants completed telephone interviews and provided urine specimens through the mail for chlamydia testing. Urines were tested using polymerase chain reaction. RESULTS: 11,001 households were contacted, with 1532 households identified as including eligible women; telephone interviews were completed, with 979 women giving a response rate of 64%. Six hundred and fifty-seven women provided a urine specimen with a response rate of 43%. Among sexually active women aged 18-24 years, the chlamydia prevalence was 3.7% (95% CI: 1.2%, 8.4%) and 0.2% (95% CI: 0.0%, 1.1%) among 25-35 year olds. Chlamydia prevalence increased significantly with an increasing number of male sexual partners. CONCLUSIONS: This is the first study of its kind in Australia and shows that chlamydia prevalence increases with an increasing number of male sexual partners in the last 12 months. Mailed urine specimens are feasible for conducting population-based chlamydia-prevalence surveys but it is difficult to obtain high response rates with this methodology. Public health resources should now be directed towards investigating how to reach young women at increased risk of infection, ensuring that they are tested for chlamydia.