Among 976 men who have sex with men (MSM) who had undergone repeat HIV testing between 1984 and 2003 at a sexually transmitted infection clinic in Rome, Italy, we observed a dramatic increase in HIV incidence in 2002 and 2003, with the cumulative incidence for 2000-2003 being twice as high as that for 1984-1995, and significantly higher than that for 1996-1999. This trend suggests the need for interventions aimed at encouraging behavioural changes among MSM.