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This prospective study aimed to study the composition and structure of the vaginal microbiota prior to Chlamydia trachomatis infection.A nested case–control study was performed in 122 women, half of which acquired C. trachomatis within a year after their first visit. At the first visit, the composition and structure of vaginal microbial communities were analysed using 16S rRNA sequencing in the context of the sociodemographic and sexual risk behaviour information using logistic regression.Five vaginal community state types (CSTs) were identified. Four CSTs were dominated by Lactobacillus spp., of which L. crispatus (37%) and L. iners (33%) were the most common. One CST was characterised by the absence of Lactobacillus spp. (25%) and the presence of an array of strict and facultative anaerobes. Multivariate logistic regression analysis revealed that women with a L. iners-dominated CST had an increased risk of C. trachomatis infection (p=0.04; OR: 2.6, 95% CI 1.0 to 6.6).The distribution of CSTs dominated by Lactobacillus spp. agreed with previous studies. However, the frequency of dysbiosis among Caucasian women was relatively high (24%). Having vaginal microbiota dominated by L. iners was associated with an increased risk for C. trachomatis infection. Therefore, we hypothesise that specific signatures of vaginal microbiota are indicative of increased host predisposition to acquiring STIs.