A nested case-control study of invasive and in situ cervical cancer was performed within a community-based cohort of 13,595 Taiwanese women assembled in 1991, with a follow-up period of 9 years. Baseline serum or plasma samples were analysed for antibodies against human papillomavirus (HPV) types 6, 16 and 18 and Chlamydia trachomatis. In total, 114 cases (42 incident cases identified during follow-up and 72 prevalent cases identified at baseline) and 519 matched controls were included in the study. HPV-16 seropositivity was strongly associated with cervical cancer (OR=6.33; 95% CI 3.45-11.62). Overall, C. trachomatis was not associated with cervical cancer, but was associated with cervical cancer in analyses restricted to incident cases of cancer (OR=2.94; 95% CI 1.17-7.42) or to cases in which serum samples were analysed (OR=3.13; 95% CI 1.16-8.47). An antagonistic interaction between HPV-6 and -16 was found in a multiplicative model. These results suggest that different HPV types might interfere in cervical carcinogenesis and that C. trachomatis is associated with cervical cancer in prospective studies, and support the notion that HPV-16 seropositivity is strongly associated with cervical cancer.