Objectives Re-infection with chlamydia may increase subsequent reproductive morbidity in women. The authors sought to identify characteristics associated with re-infection.

Methods A cohort of all women aged 10–49 years with a notification of genital chlamydia in the Australian state of New South Wales during 1999–2008 was defined. Probabilistic linkage was used to identify women with repeat notifications in the same period. The risk of repeat notification was examined according to age and other characteristics using proportional hazards regression.

Results Among 40 936 women in the cohort, 3236 had at least one repeat chlamydia notification over an average of 3.5 years of follow-up. The incidence of repeat notification was greatest in the first year after index notification (4.5 per 100 person-years) and decreased thereafter. The RR of repeat notification increased by 8% (95% CI 7% to 9%) for each year decrease in age. Compared with women aged 20–21 years at index chlamydia notification, women aged <16 years were twice as likely to have a repeat notification (adjusted HR 2.12, 95% CI 1.75 to 2.56), while women aged 26–27 years were half as likely (adjusted HR 0.53, 95% CI 0.43 to 0.66). Year of index notification, parity and concurrent or past gonorrhoeal infection were also significantly associated with the risk of repeat notification, but socioeconomic status and area of residence were not.

Conclusions Younger age is a strong predictor of chlamydia re-infection in women. The results support targeting interventions to prevent re-infections to very young women.