Since the late 1990s, there has been a resurgence of infectious syphilis, with notable outbreaks in Brighton, Manchester, London and Dublin, predominantly among men who have sex with men (MSM). We report a similar outbreak in Northern Ireland. Genitourinary (GU) medicine clinic attendees were assessed from 1 July 2000 to 30 June 2005 to identify those who met the agreed criteria for primary, secondary or early latent syphilis. In total, 161 individuals were diagnosed with syphilis and 121 were MSM. Sixteen individuals indicated a contact in Dublin as the likely source of infection. Thirty were identified through contact tracing. Over half contracted the infection through oral intercourse. Most (106) had one or two partners in the previous three months. Twelve cases were HIV positive (nine were aware of their status at the time of presumed infection). In conclusion, initially, cases acquired their infection in Dublin and, as the outbreak gained momentum, syphilis was contracted within Northern Ireland. The cohort was not generally associated with a high number of sexual contacts, multiple anonymous partners or specific locations. The challenge is to educate both patients and health-care professionals to sexual health issues; specifically, the risk associated with casual oral sex by MSM.