The aim of this retrospective study was to determine the prevalence of Chlamydia trachomatis co-infection in men with gonorrhoea attending a sexually transmitted diseases clinic in Edinburgh, Scotland. During the study period, there were 660 cases of culture-proven gonorrhoea. Chlamydial DNA was detected in the urethra in 79 (31%; 95% confidence interval [CI], 25-37%) heterosexual men who have sex with women (MSW); the median age was significantly lower than those with gonorrhoea alone (24.0 versus 30.0; P <0.0005). The prevalence of urethral chlamydial infection among MSW was significantly higher than among men who have sex with men (MSM) (32 [12%; 95% CI, 8-16%] of 268 MSM) (chi2 = 27.21; P <0.001). Sixteen (24%; 95% CI, 14-34%) of 68 MSM with rectal gonorrhoea had concurrent rectal chlamydial infection. The high prevalence of concurrent gonorrhoea and chlamydiae therefore warrants empirical treatment and/or testing for chlamydia in all men with urethral gonorrhoea.