['To determine prevalence and incidence of bacterial vaginosis (BV) and risk factors in young sexually-active Australian women.', '1093 women aged 16-25 years were recruited from primary-care clinics. Participants completed 3-monthly questionnaires and self-collected vaginal smears 6-monthly for 12-months. The primary endpoint was a Nugent Score\u200a=\u200a7-10 (BV) and the secondary endpoint was a NS\u200a=\u200a4-10 (abnormal flora [AF]). BV and AF prevalence estimates and 95% confidence intervals (95%CI) were derived, and adjusted odds ratios (AOR) calculated to explore epidemiological associations with prevalent BV and AF. Proportional-hazards regression models were used to examine factors associated with incident BV and AF.', 'At baseline 129 women had BV [11.8% (95%CI: 9.4-14.2)] and 188 AF (17.2%; 15.1-19.5). Prevalent BV was associated with having a recent female partner [AOR\u200a=\u200a2.1; 1.0-4.4] and lack of tertiary-education [AOR\u200a=\u200a1.9; 1.2-3.0]; use of an oestrogen-containing contraceptive (OCC) was associated with reduced risk [AOR\u200a=\u200a0.6; 0.4-0.9]. Prevalent AF was associated with the same factors, and additionally with >5 male partners (MSP) in 12-months [AOR\u200a=\u200a1.8; 1.2-2.5)], and detection of C.trachomatis or M.genitalium [AOR\u200a=\u200a2.1; 1.0-4.5]. There were 82 cases of incident BV (9.4%;7.7-11.7/100 person-years) and 129 with incident AF (14.8%; 12.5-17.6/100 person-years). Incident BV and AF were associated with a new MSP [adjusted rate ratio (ARR)\u200a=\u200a1.5; 1.1-2.2 and ARR\u200a=\u200a1.5; 1.1-2.0], respectively. OCC-use was associated with reduced risk of incident AF [ARR\u200a=\u200a0.7; 0.5-1.0].'] Are prevalent and incident bacterial vaginosis associated with sexual and contraceptive behaviours in young Australian women?