OBJECTIVES:To assess the effect of early syphilis on HIV viral load (VL) and CD4 cell count in patients with HIV and to analyze factors associated with changes in HIV VL and CD4 cell count. DESIGN:Multicenter study of a series of patients with HIV who were diagnosed with early syphilis infection during 2004 through 2005. Patients who started or changed their highly active antiretroviral therapy (HAART) regimen during the analysis period were excluded. RESULTS:One hundred eighteen patients were analyzed95.8% were men, mean patient age was 38.2 years, 83.9% were homosexual men, 50.8% were on antiretroviral therapy at the time syphilis was diagnosed, and HIV and syphilis diagnoses were coincident in 38 (32.2%) cases. CD4 cell counts were lower during syphilis than before (590 vs. 496 cells/μL; P = 0.0001) and after syphilis treatment (509 vs. 597 cells/μL; P = 0.0001). The HIV VL increased in 27.6% of patients during syphilis. The only factor associated with an HIV VL increase was not being on HAART, and the only factor associated with a CD4 count decrease >100 cells/μL during syphilis was the prior CD4 cell count. CONCLUSIONS:Syphilis infection was associated with a decrease in the CD4 cell count and an increase in the HIV VL in almost one third of the patients. In this series, more than two thirds of the syphilis cases were diagnosed in patients who were previously known to be infected with HIV.