A serologic test for antibodies to chlamydia may be a useful tool for trachoma surveillance. However, little is known about the longitudinal stability of antibody status, especially following Mass Drug Administration (MDA), which is critical to understanding serostatus in trachoma-endemic areas. A longitudinal cohort of 1908 children ages 1-9 years in Tanzania from 50 communities were followed at baseline and for 6 months after MDA. They were evaluated for clinical trachoma, conjunctival swabs were tested for chlamydial infection using GeneXpert platform, and blood spots were collected on filter paper and dried to test for antibodies to Chlamydia trachomatis pgp3 using the Luminex platform. 6.3% of children in the study had infection, and coverage with MDA was 97%. 670 (35%) were sero-positive for pgp3 antibodies at baseline, and 4.0% of these seroreverted to negative following MDA. Of those seronegative at baseline, 3.6% seroconverted. The individual change in log median fluorescence intensity(MFI-BG) values was -0.15 overall (p < .001). Seroconversion rates were lower following MDA and seroreversion rates were slightly higher compared to rates in this same cohort in the absence of MDA. MDA has a small effect on reduction of MFI-BG.