The adequacy of treatment for syphilis has routinely been evaluated by the serological response, i.e. the rapid plasma reagin test (RPR). Since the description of AIDS and HIV aspects of both the natural history of syphilis and the response of Treponema pallidum to treatment have come under increased scrutiny. With concurrent epidemics of HIV and syphilis in New York City, a serological case-control study was done to determine whether HIV-infected individuals given treatment for primary or secondary syphilis have a modified serological response. All study participants had primary or secondary syphilis and paired specimens available for testing. Cases were defined as people who were HIV-positive and were compared with controls who were HIV-negative. HIV-infected patients with primary syphilis when compared with HIV-negative controls were less likely to have a fourfold or greater RPR decrease or seroreversion within 6 months of treatment [15 out of 28 versus 153 out of 210; odds ratio = 0.4, P less than 0.05]. Cases and controls with secondary syphilis had similar serological responses after treatment for syphilis. Although this study adds to the growing body of literature which suggests that HIV may alter the RPR response, prospective studies are needed to determine definitively whether HIV alters the serological response to therapy in patients with early syphilis.