Over the past five years, a series of syphilis outbreaks mainly occurring among gay men have been observed in Europe. One of these outbreaks was reported in the city of Antwerp, Belgium, during the first quarter of 2001. This outbreak is still ongoing in 2004. Furthermore, active syphilis diagnoses reported by the Sentinel Laboratory Network rose by 89% in the country during the fourth quarter of 2003. An increase in Brussels was also observed during the same quarter (+300%; 24 cases reported). Overall, the sentinel network of clinicians reported that 93.4% of patients were male; among them, 79.9% were men having sex with men (MSM). The overall proportion of patients co-infected with HIV was 50.5% (MSM: 58.6%; male heterosexuals: 23.8%; females: 8.3%); 76.1% of co-infected patients were already aware of their HIV infection at the time they were diagnosed with syphilis.