During the summer of 1999, an outbreak of early syphilis among men who have sex with men (MSM) who met their sex partners on the Internet presaged a rapidly expanding syphilis epidemic in San Francisco. By 2002, San Francisco had the highest rates of primary and secondary syphilis of any metropolitan area in the United States. During 1998-2002, the number of early syphilis cases increased, from 41 cases in 1998 to 495 cases in 2002. Concomitant with the increase in early syphilis was an increase in the proportion of cases among MSM, from 22% in 1998 to 88% in 2002. To assess the association between early syphilis infection and use of the Internet by MSM to meet sex partners, the San Francisco Department of Public Health (SFDPH) analyzed surveillance data and case reports. This report summarizes the results of that analysis, which suggest that public health officials might find the Internet to be an important tool for 1) promoting disease awareness, prevention, and control and 2) accessing sex partners of syphilis patients to conduct appropriate partner notification, evaluation, and management. The findings underscore the need for public health officials to understand the role of the Internet in facilitating the spread of sexually transmitted diseases (STDs), including the human immunodeficiency virus (HIV). With the assistance of community partners, other jurisdictions can examine the online social/sexual networks that are used commonly in their gay and bisexual communities and develop an effective means of communicating prevention and control messages online.