Belgium is currently experiencing an upward trend in the number of new HIV diagnoses characterised by a continuous increase in the number of cases among men who have sex with men (MSM). Based on surveillance data, in the past decade the yearly number of newly diagnosed HIV cases in MSM increased more than threefold, from 101 cases diagnosed in 1999 to 332 cases in 2008. During this period, the majority of new HIV infections in MSM were diagnosed among Belgian citizens (72%), followed by other European nationalities (13%). The increase in HIV diagnoses does not reflect an increase in HIV testing since the number of tests performed nationwide remained remarkably stable over time. The steady increase in the number of newly diagnosed HIV cases among MSM, and the high proportion of MSM among HIV-positive patients co-infected with other sexually transmitted infections (STI) (95.6% in 2008) indicate increases in unsafe sex practices in this group. Development of behavioural surveillance and more qualitative research on reasons for unsafe sex are needed in order to develop more effective prevention strategies.