The aim of this study was to examine sexual behaviour, condom use and rates of sexually transmitted infections (STIs) among attendees at a dedicated on-site STI clinic at a South London HIV centre. Data were prospectively collected by using a nurse-completed questionnaire. Ninety-eight percent of women reported one or no sexual partners in the preceding three months, whereas 57% of men who have sex with men (MSM) reported two or more partners. Only 28% of women, 53% of heterosexual men and 29% of MSM always used a condom for vaginal or anal intercourse. Positive STI diagnoses were found in 17.5% of women, 20% of heterosexual men and 49% of MSM. Twenty percent of patients who reported always using a condom and 38% of MSM reporting no sexual activity in the preceding three months had an STI. These results highlight the need for safe sex promotion and STI screening in HIV-infected patients regardless of self-reported sexual activity.