Background: In England, screening for genital chlamydial infection has begun; however, screening frequency for women is not yet determined. Aim: To measure chlamydia incidence and reinfection rates among young women to suggest screening intervals. Methods: An 18-month prospective cohort study of women aged 16-24 years recruited from general practices, family planning clinics and genitourinary medicine (GUM) clinics: baseline-negative women followed for incidence and baseline-positive women for reinfection; urine tested every 6 months via nucleic acid amplification; and behavioural data collected. Extra test and questionnaire completed 3 months after initial positive test. Factors associated with infection and reinfection investigated using Cox regression stratified by healthcare setting of recruitment. Results: Chlamydia incidence was mean (95% CI) 4.9 (2.7 to 8.8) per 100 person-years (py) among women recruited from general practices, 6.4 (4.2 to 9.8) from family planning clinics and 10.6 (7.4 to 15.2) from GUM clinics. Incidence was associated with young age, history of chlamydial infection and acquisition of new sexual partners. If recently acquiring new partners, condom use at last sexual intercourse was independently associated with lower incidence. Chlamydia reinfection was mean (95% CI) 29.9 (19.7 to 45.4) per 100/ person-year from general practices, 22.3 (15.6 to 31.8) from family planning clinics and 21.1 (14.3 to 30.9) from GUM clinics. Factors independently associated with higher reinfection rates were acquisition of new partners and failure to treat all partners. Conclusions: Sexual behaviours determined incidence and reinfection, regardless of healthcare setting. Our results suggest annual screening of women aged 16-24 years who are chlamydia negative, or sooner if partner change occurs. Rescreening chlamydia-positive women within 6 months of baseline infection may be sensible, especially if partner change occurs or all partners are not treated.