Chlamydia trachomatis is the most common bacterial sexually transmitted infection (STI) in the UK. The Department of Health set up an opportunistic screening programme for genital chlamydia infection, focusing on sexually active 16-24-year-old women and some men. This study identified those patients re-attending the genitourinary medicine (GUM) clinic and followed them up until September 2002. We examined the reasons for attendance and re-infection with chlamydia. Two hundred and eighty-five patients re-attended the clinic. Two-thirds of these had changed their sexual partners in the follow-up period. Fifty-six patients were diagnosed with genital chlamydia infection in subsequent clinic visits. The majority of them had changed their sexual partners, suggesting new acquisition of infection. This suggests that in this age group re-screening should be offered at a year interval. Patients diagnosed with genital chlamydia infection should be referred to the GUM clinic for further STI screening and partner notification.