Resistance of Neisseria gonorrhoeae (GC) to azithromycin is increasing. The dose of azithromycin used for treating uncomplicated genital chlamydia may be subtherapeutic for GC and contribute to gonococcal resistance. The National Chlamydia Screening Programme (NCSP) does not routinely screen for GC but advises further testing of clients with symptoms. This will miss asymptomatic cases. We conducted a case-note review of 100 women aged 16-24 years diagnosed with chlamydia and women aged 16-24 years co-infected with GC and chlamydia, who attended a London genitourinary clinic from 1 October 2004 to 30 September 2005. There were 559 chlamydia diagnoses; 38 (7%) were co-infected with GC. Those co-infected were younger: mean age 18.9 versus 20.3 (P = 0.005). They were also more likely to have symptoms: 28/38 (74%) versus 50/94 (53%) (P = 0.03). Two of 10 women with asymptomatic GC were contacts; meaning that eight cases of asymptomatic GC would have received subtherapeutic treatment. In conclusion, additional testing for GC should be considered in local NCSP protocols where there is a high prevalence of both infections.