Male circumcision has been shown to reduce the risk of acquiring and transmitting a number of venereal infections. However, little is known about the association between male circumcision and the risk of Chlamydia trachomatis infection in the female partner. The authors pooled data on 305 adult couples enrolled as controls in one of five case-control studies of invasive cervical cancer conducted in Thailand, the Philippines, Brazil, Colombia, and Spain between 1985 and 1997. Women provided blood samples for C. trachomatis and Chlamydia pneumoniae antibody detection; a type-specific microfluorescence assay was used. Multivariate odds ratios were computed for the association between male circumcision status and chlamydial seropositivity in women. Compared with women with uncircumcised partners, those with circumcised partners had a 5.6-fold reduced risk of testing seropositive for C. trachomatis (82% reduction; odds ratio = 0.18, 95% confidence interval: 0.05, 0.58). The inverse association was also observed after restricting the analysis to monogamous women and their only male partners (odds ratio = 0.21, 95% confidence interval: 0.06, 0.72). In contrast, seropositivity to C. pneumoniae, a non-sexually-transmitted infection, was not significantly related to circumcision status of the male partner. These findings suggest that male circumcision could reduce the risk of C. trachomatis infection in female sexual partners.