BACKGROUND: Serologic tests for syphilis results at the time of diagnosis are the basis for evaluating response to syphilis therapy. After treatment, however, serologic tests for syphilis titers may continue to increase for several weeks. We evaluated rapid plasma reagin (RPR) titer variation during the 14 days after therapy using data from a recent large, prospective randomized controlled trial. METHODS: Prospectively enrolled participants in North America and Madagascar with primary, secondary, or early latent syphilis were randomly assigned to penicillin, doxycycline (in the case of penicillin allergy), or azithromycin treatment. Blood for RPR analysis was drawn at days 0, 7, and 14 posttreatment. All RPR titers were determined simultaneously at a central laboratory. RESULTS: Four hundred and seventy patients had data available for at least 2 of 3 RPR measurements. Overall, 20% of patients showed a titer increase of at least 1 dilution in the 14 days after therapy. The greatest proportion of titer increases following therapy was observed in patients with primary syphilis. Comparing outcome of therapy using the initial (day 0) RPR titer versus the maximal RPR titer (during 14 days) resulted in outcome reclassification in 2.98% of participants. CONCLUSIONS: Despite the fact that about 20% of early syphilis patients had increases in RPR titers immediately after treatment, these changes rarely influenced assessment of therapeutic outcome. Only 3% of patients treated would have been reclassified. Copyright © 2012 American Sexually Transmitted Diseases Association.