Chlamydia trachomatis IgA and IgG antibody titers were determined by indirect immunoperoxidase assay in sera of 88 female patients with C. trachomatis-positive cervicitis and 55 C. trachomatis negative healthy controls. Serial serum samples obtained until 66 weeks after treatment were also studied for IgA and IgG antibodies in 28 of the 88 patients with chlamydial cervicitis. In the 88 patients, the presence (94.3%) of antibody (titer greater than or equal to 16) was comparable to that (100%) of IgG antibody (titer greater than or equal to 32), but in healthy controls the presence (5.4%) of IgA antibody was lower than that (30.6%) of IgG antibody. Serum IgA antibody decreased gradually and was undetectable (titer less than or equal to 8) at 20 weeks after treatment in 78.6% of the 28 patients. Serum IgG antibody persisted at stable levels within the same period. These data suggest that the presence of serum IgA antibody to C. trachomatis is correlated with active C. trachomatis infection of the female genital tract.