An infection with Treponema pallidum does not confer solid immunity to reinfection. Consequently, it is not unusual for the physician to see a patient with reinfection syphilis. In this study there were 36 patients who, after having an initial infection with T. pallidum, became reinfected with the primary, secondary, or latent stage of syphilis. The titers of antibody to T. pallidum during reinfection were consistently higher than those during the first infection, and the serologic responses to treatment were always slower.