This paper describes a serological survey applied to 3098 female commercial sex workers, in order to determine the prevalence of syphilis. The women attend regularly an AIDS orientation center sponsored by CONASIDA in Mexico City. The laboratory tests included the rapid plasma reagin assay (presumptive analysis) and the fluorescent treponemal antibody-absorption technique (confirmatory test). No primary syphilis cases were detected and the prevalence of latent asymptomatic syphilis was 8.2% (255/3098). The sensitivity and specificity of the presumptive test compared to the confirmatory test were 74% and 98% respectively, and the positive predictive value was 77% and the negative predictive value 98%. All seropositive women received the standard penicillin treatment for syphilis. Considerations of importance are offered regarding interpretation and usefulness of this kind of research.