Because syphilis infection facilitates acquisition and transmission of human immunodeficiency virus (HIV), recent outbreaks of syphilis among men who have sex with men (MSM) in major U.S. cities, including San Francisco and Los Angeles, and reported increases in sexual risk behavior have raised concerns about potential increases in HIV transmission. In 2002, MSM accounted for the majority of primary and secondary (P&S) syphilis cases in men reported in San Francisco (93%) and Los Angeles (81%). To investigate a potential change in HIV incidence associated with the syphilis outbreaks in the two cities, local, state, and federal health officials analyzed data from HIV counseling and testing centers and a municipal sexually transmitted disease (STD) clinic. This report describes the results of that investigation, which indicated that, as of 2002, the outbreaks of syphilis had not had a substantial impact on HIV incidence among MSM in these two cities. However, the continued increase in syphilis cases in MSM underscores the need for integrated HIV- and STD-prevention strategies to control syphilis outbreaks and prevent potential increases in HIV infections (6,7) and for further systematic studies of HIV incidence among MSM infected with syphilis.