We investigated prevalence of sexually transmitted infections (STI) in a cohort of HIV-1-infected pregnant women and described factors associated with STI diagnosis, as a nested study within the European Collaborative Study (ECS). The ECS is a cohort study in which HIV-infected pregnant women are enrolled and their children followed from birth, according to standard clinical and laboratory protocols. Information on STIs diagnosed during pregnancy was collected retrospectively from the antenatal records of women enrolling between January 1999 and October 2005; other variables were obtained from the ECS prospective database. A total of 1,050 women were included: 530 in Western Europe and 520 in Ukraine. Syphilis was the most common bacterial STI (2% prevalence, 95% CI 1.2-3.0). Prevalence of HPV-related genital lesions was 8.6% (95%CI 6.9-10.4) and prevalence of Trichomonas vaginalis was 12.1% (95%CI 10.2-14.2). Women in Ukraine (AOR 10.7, 95%CI 3.7-30.5), single women (AOR 3.9, 95%CI 1.2-12.7), sexual partners of injecting drug users (AOR 3.8, 95%CI 1.4-10.4) and women with CD4 counts <200 cells/mm(3) (AOR 5.4, 95%CI 1.0-28.1) were at increased risk of diagnosis with Chlamydia trachomatis, syphilis or Trichomonas vaginalis. African origin (AOR 1.9, 95%CI 1.1-3.3) and CD4 count <200 cells/mm(3) (AOR 3.4, 95%CI 1.5-7.8) were associated with HSV-2 and/or HPV-related genital lesions. Antenatal screening should be considered an effective tool for diagnosis, treatment and prevention of further transmission of STIs. HIV-infected women should receive adequate screening for STIs during pregnancy together with appropriate counseling and follow-up for treatment and prevention.