The aim of this study was to investigate the characteristics of patients co-infected with
Chlamydia trachomatis
and
Neisseria gonorrhoea
. A retrospective case-control study was performed, which included 399 co-infected patients seen at a sexually transmitted infection clinic in Copenhagen, Denmark. Case-control groups included 300 patients who tested positive only for
N. gonorrhoea
, 300 who tested positive only for
C. trachomatis
, and 300 who tested negative for both
N. gonorrhoea
and
C. trachomatis
in the same study period. For men, non-Danish origin (odds ratio (OR) 2.3, 95% confidence interval (Cl) 1.34–4.12), previous sexually transmitted infections with
C . trachomatis
(OR 3.3, 95% CI 1.94–5.92) and
N. gonorrhoea
(OR 10.6, 95% CI 6.36-17.76), and higher number of sex partners (OR 1.7, 95% Cl 1.40-2.28) were significantly associated with diagnosis of co-infection. For women, previous sexually transmitted infections with
C. trachomatis
(OR 6.7, 95% CI 3.89-11.78) and N. gonorrhoea (OR 10.4, 95% CI 4.99-21.71), and higher number of sex partners (OR 1.8, 95% CI 1.28-2.56) were significantly associated with a diagnosis of co-infection, whereas being of non-Danish origin was, in some cases, a protective factor (OR 0.3, 95% CI 0.17-0.69). Furthermore, this study demonstrated sex-associated characteristics that should raise concern about co-infection, including: for men, being of non-Danish origin, men who have sex with men status, and higher age, and, for women, young age, in particular, and previous sexually transmitted infections.