This thesis presents data on incidence and risk factors for individual sexually transmissible infections (STIs) and STIs as risk factors for HIV acquisition in a community-based cohort of HIV negative homosexual men in Sydney. Nearly half of men aged under 25 years were seronegative to hepatitis A and B infection. The prevalence of hepatitis C (HCV) was 0.85%, which was close to that of the general population. HCV infection was strongly associated with injecting drug use (OR 60.43, 95% CI 6.70-544.79), and sexual transmission was not demonstrated in this cohort. There was a nearly 40 fold increase in syphilis notifications in inner Sydney between 1999 and 2004. The stable incidence of 0.6% per year in the HIM cohort suggests that it was disproportionately affecting HIV positive men. Oral sex was an important transmission route and about one third of cases were asymptomatic. The incidence of urethral and anal gonorrhoea was 3.78 and 3.19 per 100 person-years, and for urethral and anal chlamydia it was 7.98 and 5.20. In addition to unprotected anal intercourse (UAI), insertive oral sex was related to urethral infections and anal infections were associated with non-intercourse anal sexual practices. The prevalence of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) was 75% and 23% respectively. Sexual contact not only with men, but with women, was significantly associated with both infections. Among those susceptible at baseline, the incidence for HSV-1 and HSV-2 was 5.58 and 1.45 per 100 person-years. Oral sex was associated with incident HSV-1 infection and certain non-intercourse anal sexual practices were significantly associated with incident HSV-2 infection. HIV incidence was 0.94 per 100 person-years. It was significantly associated with a higher number of episodes of receptive UAI with a partner of unknown HIV status (p trend&lt0.001) or a partner known to be HIV positive (p trend&lt0.001). After controlling for sexual behaviour, HIV seroconversion was significantly associated with anal gonorrhoea (HR 12.68, 95% CI 3.66-44.00). The association with anal warts and prevalent HSV-1 infection was of borderline significance. These data will inform intervention designs targeting STIs which aim to prevent HIV in homosexual men.