Cervical carcinoma is a sexually transmitted disease most strongly linked with human-papillomavirus (HPV) infection. We conducted a prospective sero-epidemiologic study to evaluate the role of Chlamydia trachomatis infection in the development of cervical carcinoma, with invasive cancer as an end point. A nested case-control study within a cohort of 530000 Nordic women was performed. Linking data files of 3 Nordic serum banks and the cancer registries of Finland, Norway and Sweden identified 182 women with invasive cervical carcinoma diagnosed during a mean follow-up of 5 years after serum sampling. The serum samples of the cases and matched cancer-free controls were analyzed for IgG antibodies to C. trachomatis, C. pneumoniae (a control microbe) and HPV types 16, 18 and 33, as well as for serum cotinine (an indicator of tobacco smoking). Serum antibodies to C. trachomatis were associated with an increased risk for cervical squamous-cell carcinoma (HPV-and smokingadjusted OR, 2.2; 95% CI, 1.3-3.5). The association remained also after adjustment for smoking both in HPV16-seronegative and -seropositive cases (OR, 3.0; 95% CI, 1.8-5.1; OR, 2.3, 95% CI, 0.8-7.0 respectively). No such association was found for C. pneumoniae. Our prospective study provides seroepidemiologic evidence that infection with C. trachomatis confers an increased risk for subsequent development of invasive squamous-cell carcinoma of the uterine cervix. Int.