Of the 228 hospitalized patients who had acute salpingitis, 69 (30%) harbored Chlamydia trachomatis in the lower genital tract (LGT). The mean antichlamydial IgG immunofluorescence antibody titer was 264 in sera from the C. trachomatis culture-positive patients but only 49 in culture-negative patients. In paired sera obtained from 167 of the patients, 32% of the culture-positive patients and 13% of the culture-negative patients showed significant titer change. Patients with abscess formation had higher mean antichlamydial antibody titer than the group with no abscess (192 vs. 53), although there was no difference between the groups in positive chlamydial cultures from the LGT (33% vs. 29%). Overall, this study indicates that C. trachomatis is a common etiologic agent in acute salpingitis, emphasizing the need to treat chlamydial cervicitis before this complication develops.