ObjectivesWe estimate the prevalence of sexually transmitted infection (STI) among patients after sexual assault, assess the possible value of azithromycin prophylaxis, and identify risk factors for assault-related STI and for not presenting at follow-up.DesignProspective observational cohort study.SettingSexual assault centre in Oslo, Norway.Participants645 patients, 602 (93.3%) women and 43 (6.7%) men, attending the centre from May 2017 to July 2019.Outcome measuresMicrobiological testing at the primary examination and at follow-up consultations after 2, 5 and 12 weeks. Estimated relative risk for assault-related STI and for not presenting at follow-up.ResultsAt primary examination, the prevalence of genital chlamydia was 8.4%,Mycoplasma genitalium6.4% and gonorrhoea 0.6%. In addition, the prevalence of bacterial STI diagnosed at follow-up and possibly from the assault was 3.0% in total: 2.5% forM. genitalium,1.4% for genital chlamydia and 0.2% for gonorrhoea. This prevalence did not change when azithromycin was no longer recommended from January 2018. There were no new cases of hepatitis B, hepatitis C, HIV or syphilis. We found no specific risk factors for assault-related STI. Patients with previous contact with child welfare service less often presented to follow-up (relative risk (RR) 2.0 (95% CI 1.1 to 3.5)), as did patients with a history of sex work (RR 3.6 (1.2 to 11.0)) or substance abuse (RR 1.7 (1.1 to 2.7)).ConclusionsMost bacterial STIs were diagnosed at the primary examination, hence not influenced by prophylaxis. There was no increase in bacterial STI diagnosed at follow-up when azithromycin prophylaxis was not routinely recommended, supporting a strategy of starting treatment only when infection is diagnosed or when the patient is considered at high risk. Sex work, substance abuse and previous contact with child welfare services were associated with not presenting to follow-up.Trial registration numberClinicalTrials.gov Registry (NCT03132389).