Background: Free general practice (GP) sexual health visits for registered adolescents have been introduced in parts of New Zealand with the aim of improving provision of primary sexual health care. Published evidence of the effectiveness of such health care interventions, particularly around any impact on uptake of testing and detection of Chlamydia trachomatis, is limited. Methods: In 2003–2004, additional funding enabled 20 practices in Waikato, New Zealand to offer free sexual health consultations for registered under-25 year olds. Practice selection was non-random and biased towards lower socioeconomic, Māori and rural populations. Registered population data were linked to laboratory testing for C. trachomatis from January 2003 to December 2005. Twenty-nine practices without additional funding served as controls. Results: Chlamydia testing among under-25 year olds at the 20 intervention practices increased over time, in contrast to non-intervention practices, with coverage of females aged 18–24 years within the intervention increasing from 13.9% in 2003, to 15.5% during the roll-out phase and to 16.8% in 2005. Intervention practices had higher test positivity rates than non-intervention practices (8.7% v. 5.9%, P < 0.01) with increases in test positivity, from 7.7% in 2003 to 10% in 2005, relating mainly to increases in positive tests among females aged less than 25 years. There was no increase in testing or detection among those aged 25 years and older at intervention practices. Conclusions: Introducing free GP visits for under-25 year olds living in rural and lower socioeconomic areas in New Zealand was associated with a significant increase in testing and detection for C. trachomatis in the target age group. This observational intervention supports the ongoing provision of free adolescent primary sexual health care.