Chlamydia trachomatis infection was examined as a cause of invasive cervical cancer (ICC) among women with human papillomavirus (HPV) infection. In total, 499 women with incident ICC (ICC patients) and 539 control patients from São Paulo, Brazil, and Manila, the Philippines, were included. C. trachomatis antibodies were detected by microimmunofluorescence assay. Presence of HPV DNA in cervical specimens was determined by a polymerase chain reaction-based assay. C. trachomatis seropositivity was associated with sexual behavior but not with HPV infection. C. trachomatis increased the risk of squamous cervical cancer among HPV-positive women (odds ratio, 2.1; 95% confidence interval, 1.1-4.0). Results were similar in both countries. There was a suggestion of increasing squamous cancer risk with increasing C. trachomatis antibody titers. This large study examined C. trachomatis and cervical cancer, taking into account the central role of HPV infection. C. trachomatis infection was found to be a possible cofactor of HPV in the etiology of squamous cervical cancer, and its effect may be mediated by chronic inflammation.