Objective: Adolescent and young adult women in urban, socioeconomically disadvantaged areas are at high risk of contracting sexually transmitted infections (STIs). We assessed associations of Chlamydia trachomatis (CT) infection with both traditional STI risk factors, and partner and partnership-related factors among low-income women in Lima, Peru, by age group. Methods: In a cross-sectional analysis of CT infection among 1290 postpartum women, cervical swabs were collected for CT polymerase chain reaction (PCR) within 48 h after delivery, and a structured interview was completed. Multivariate logistic regression was used to evaluate risk factors for CT, with separate models stratified by age: adolescents (12–19 years), young women (20–24 years), and older women (≥25 years). Results: CT was detected in 9.6% of adolescents, 9.0% of young women, and 5.4% of older women (p = 0.03). Among adolescents, history of drug use (odds ratio [OR] = 5.62, 95% confidence interval [CI] 1.03–30.6) and short duration of current partnership (OR = 2.6, 95% CI 1.14–5.93) were the strongest predictors of CT infection. Among young women, younger age at coitarche (OR = 0.74 for each year older, 95% CI 0.60–0.91) and low income (OR = 2.40, 95% CI 1.04–5.55) were associated with CT, while self-report of ever using condoms was protective (OR = 0.22, 95% CI 0.08–0.61). Among older women, only younger age at coitarche was related to CT (OR = 0.85, 95% CI 0.75–0.97). Conclusions: Risk factors for CT among women in Lima, Peru, differed for adolescents, young women, and older women, which may reflect differences in biology and/or immunology of CT as well as variability in the occurrence of specific risk behaviors by age group. [ABSTRACT FROM AUTHOR]