Culture and serology studies have shown Chlamydia trachomatis (CT) to be one of the causes of acute salpingitis (AS). In the present investigation, results of cervical cultures were correlated with serum antibody titers to CT in patients with laparoscopically verified AS. Serum samples from 206 patients, including paired sera from 80, were assayed. Of 206 patients, 118 had chlamydial IgG antibody titers of 1:64 or more. Patients with negative cultures for CT and an IgG titer of 1:64 or more had a significantly higher geometric mean titer than corresponding patients with positive cultures. In paired sera, a seroconversion or a fourfold or greater rise in IgG titer to CT was demonstrated in 35%, while a further 11% had detectable IgM antibody in a titer of 1:8 or more. The overall isolation frequency of CT was 33%, compared with 19% for Neisseria gonorrhoeae.