OBJECTIVE: To assess trends in sexually transmitted infection (STI) prevalence and HIV incidence and associated factors in a cohort of female barworkers exposed to behavioural interventions and STI screening and treatment. METHODS: An open cohort of 600 female barworkers in Mbeya Region, Tanzania was offered 3-monthly information and education sessions on HIV/STI and reproductive health, voluntary HIV counselling and testing and clinical health check-ups including STI syndromic management with simple STI laboratory support. Outcome assessments included HIV, herpes simplex virus type 2 (HSV-2) and syphilis serology, polymerase chain reaction for Neisseria gonorrhoeae, Chlamydia trachomatis and ulcerative STI, microscopy for Trichomonas vaginalis, Candida albicans and bacterial vaginosis and interviews on sociodemographic and behavioural characteristics. RESULTS: Over a period of 30 months 600 barworkers were enrolled at the baseline examination round and 153 thereafter as replacements for losses to follow-up. At 3-monthly examinations the prevalence of gonorrhoea declined steadily from 22.2 to 6.8% (odds ratio for trend per quarter: 0.81; P < 0.001). The prevalence of all other STI/RTI, except for genital herpes and bacterial vaginosis, also decreased significantly. HIV incidence declined from 13.9/100 to 5.0/100 person-years over three consecutive 9-month periods. HIV incidence was significantly associated with genital ulcers and positive syphilis serology, but not with genital herpes or HSV-2 seropositivity. CONCLUSION: A relatively simple intervention consisting of regular 3-monthly STI screening and syndromic management in combination with HIV/STI information and counselling sessions was well accepted and effective in reducing STI among barworkers. Such interventions should be implemented more widely in high-risk environments in sub-Saharan Africa.