High-risk human papillomavirus (HPV) is considered the main etiological agent for cervical neoplasia. However, the presence of a single type HPV infection alone is unlikely to be sufficient to cause cervical cancer. There is epidemiologic evidence suggesting that HPV and Chlamydia trachomatis play a central role in the etiology of cervical intraepithelial neoplasia and subsequent cervical cancer. To evaluate the HPV prevalence and the seropositivity for C. trachomatis in women referred to the colposcopy clinic due to an abnormal cervical smear and to examine the effect of this association on the severity of cervical neoplasia. Following enrollment, 131 patients underwent colposcopy and biopsies when necessary. HPV DNA was detected by the polymerase chain reaction (PCR) and genotyping was performed by reverse line-blot hybridization assay. C. trachomatis seropositivity was tested by ELISA for the detection of IgG antibodies. The prevalence of HPV infection was 86.3%. Seropositivity for C. trachomatis was 26%. Thirty-one women (27.4%) were positive for C. trachomatis antibodies and HPV-DNA. The most prevalent HPV type in C. trachomatis-seropositive women were HPV 16 (51.6%) and this HPV type was present mainly in neoplasia cases. Positivity for HPV, particularly HPV types 16 and 18, and C. trachomatis seropositivity was significantly associated with a diagnosis of high grade neoplasia. Borderline significance was observed after adjustment for HPV. C. trachomatis seropositivity is associated with high grade neoplasia in women infected with HPV, mainly when the types 16 and 18 were involved. J. Med. Virol. 84: 1143–1150, 2012. © 2012 Wiley Periodicals, Inc.