Objective: To determine the incidence of sexually transmitted infections (STI) among decriminalised and regulated sex workers in Victoria. Methods: The incidence of STI was calculated for individuals who attended the Melbourne Sexual Health Centre on more than one occasion. Results of initial screen specimens were not included. Follow up time was calculated in person months and used as the denominator with the number of "specified" STIs diagnosed over the study period as the numerator. Results: Among 388 sex workers the incidence of chlamydia, Trichomonas vaginalis, genital warts, and herpes was 0.61, 0.11, 0.79, and 0.17, respectively, per 100 person months of follow up. The mean number of sexual non-paying private partners in the past 3 months was significantly greater among those with chlamydia (0.8 v 1.5, p<0.01) and any STI (0.7 v 1.2, p<0.05). Conclusion: The incidence of STIs was low among decriminalised and regulated sex work and most infections were related to partners outside of work. Frequent screening of sex workers will reduce the chance of workers passing on an STI but is expensive. However, it may also discourage women from joining the sex work system and push them into an illegal system with a worse outcome.