A cross-sectional study of new clients with either gonorrhoea or chlamydia attending King's College Hospital in 1998. One thousand two hundred and thirty-nine women and 1141 men had gonorrhoea, chlamydia or both. Overall, 24.2% (124/512) of heterosexual men and 38.5% (136/353) of women with gonorrhoea also had chlamydia (P<0.001). Of heterosexual males 18.8% (124/660) and 13% (136/1022) of females with chlamydia also had gonorrhoea (P=0.002). Ethnicity had no effect on the proportion of co-infection after controlling for age and gender. Clients with dual infection were younger than those with either infection alone (P=0.0001). Over half of women and a quarter of men aged 15 to 19 years were dually infected so testing for both gonorrhoea and chlamydia may be appropriate in this age group in settings outside genitourinary clinics. The high proportion of cases of gonorrhoea that also have chlamydia justifies the policy of epidemiological treatment for chlamydia.