Objectives: To describe healthcare settings attended by women with clinical pelvic inflammatory disease (PID), to calculate the cost of a PID episode and to estimate how many cases could be prevented in London annually at current chlamydia screening levels. Methods: An ethnically diverse sample of 2259 16-24 year old, sexually active, female London students were recruited to a chlamydia screening trial in 2004-2006 of whom 94% (2115) were followed up after 12 months for incidence of PID. A cost analysis examined healthcare settings attended by women with PID, the cost of an episode of PID and the number of cases of PID in London due to untreated chlamydia at baseline that could be prevented per year at 2009 annual screening levels. Results: Of 35 PID cases, 17 (47%) first presented in general practice, 15 (42%) at a genitourinary medicine clinic, two elsewhere and one was admitted to hospital. The average number of consultations for a PID episode was 2.0 (range 1-4) and the average cost was £163 (range £29-960). Assuming 414 345 sexually active women aged 16-24 in London, 6% chlamydia prevalence at baseline and a 7.3% difference in PID rates between screened and unscreened chlamydia positives, 391 (95% CI -44 to 882) cases of chlamydia-associated PID costing £63 733 could be prevented each year in London at 21.5% 2009 annual screening levels. Conclusions: Most women with PID were managed in the community. The number and cost of PID cases prevented by a single annual chlamydia screen is low suggesting that cost effectiveness may depend mainly on the prevention of long-term sequelae.