The aim of this article is to describe trends in infectious syphilis in the UK, and specifically the epidemiology of the London syphilis outbreak, the largest in the UK to date. Analysis of routine surveillance data from genitourinary medicine (GUM) clinics was performed as well as data collection through enhanced surveillance systems. There have been substantial increases in diagnoses of infectious syphilis between 1998 and 2003, with a 25-fold increase seen in men who have sex with men (MSM) (from 43 to 1028 diagnoses); 6-fold (138 to 860) in heterosexual men and 3-fold (112 to 338) in women. The national rise in syphilis was driven by a series of local outbreaks, the first of which occurred in 1997. To date, 1910 cases have been reported in the London outbreak, first detected in April 2001. High rates of HIV co-infection were seen among MSM, with MSM likely to be of white ethnicity and born in the UK. In contrast, heterosexuals were more likely to be of black ethnicity and born outside the UK. Most syphilis infections were acquired in London. MSM bear the brunt of the national resurgence in infectious syphilis. Substantial rises in male heterosexual cases has resulted in a divergence between male heterosexual and female cases, which now requires further investigation.