Recent outbreaks of syphilis among men who have sex with men (MSM) in major cities in the United States and reported increases in sexual risk behavior have raised concerns about potential increases in human immunodeficiency virus (HIV) transmission. The majority of MSM who have early syphilis are HIV infected; in preliminary studies, rates of recent HIV infection among them are also high. Data from San Francisco, Los Angeles, and Seattle-King County, however, suggest no temporal increases in HIV incidence among MSM seeking HIV testing at select large public sites during the syphilis outbreaks. Because most HIV incidence and behavioral data are from large metropolitan areas with large gay populations and well-established HIV epidemics, we do not know whether, nationally, incidence of HIV infection among MSM has been increasing, decreasing, or stable during syphilis outbreaks. Further studies of HIV incidence in larger and smaller cities with different maturities of HIV epidemic are warranted. Comprehensive and integrated HIV/STD prevention and control efforts are also needed to halt the spread of syphilis and reduce HIV transmission among gay and bisexual men.