Objectives: To determine trends and correlates of chlamydia positivity among young heterosexuals attending Australian sexual health services and to compare these with population-based notification data. Methods: Data from 18 sexual health services and the national notification scheme were analysed. A χ2 test assessed trends in chlamydia positivity among young heterosexuals tested from 2006 to 2010, and logistic regression was used to determine correlates of positivity. Nucleic acid amplification tests were used throughout the study period. Results: During 2006-2010, 64 588 heterosexuals aged 15-29 years attended the sexual health services for the first time and the annual chlamydia testing rate was consistently >80%. Overall, chlamydia positivity increased by 12%, by 8.3% in heterosexual men (from 13.2% in 2006 to 14.3% in 2010; p-trend=0.04) and by 15.9% in women (from 11.3% in 2006 to 13.1% in 2010; p-trend<0.01). Independent correlates of chlamydia positivity in sexual health service patients were being aged 15-24 years, residing in a regional/rural area, being Aboriginal and/or Torres Strait Islander, being a non-Australian resident and attending in 2010 compared with 2006. Over the same period, the population-based notification rate increased by 43% against a background of a >100% increase in testing. Conclusions: The sexual health service network suggests a moderate increase in chlamydia prevalence in young heterosexuals tested at sexual health services, in contrast to the steep increase shown by notifications. This highlights the caution needed in interpreting chlamydia trends without a corresponding testing denominator.