After an initial peak in the mid-1980s, HIV incidence in men who have sex with men (MSM) declined in most western industrialised countries and then levelled off during the 1990s. Since the late 1990s, increasing numbers of newly diagnosed HIV infections in MSM have been observed in the majority of countries with large and visible MSM communities. Based on a review of national and international behavioural surveillance studies of MSM and national HIV surveillance data, we propose a model for the HIV epidemic in MSM in Germany. The model includes aspects such as individuals' increasing numbers of sexual partners and increasing frequency of unprotected anal intercourse, conditional condom use based on real or perceived HIV status of sexual partners (HIV "serosorting") and sexual role assignments (insertive versus receptive based on HIV status (HIV "seropositioning"), selection of partners and formation of sexual networks through seeking sexual partners on the internet, the introduction of HAART and changing HAART treatment strategies. All these aspects have been shown or are suspected to increase or decrease HIV transmission risk in MSM. We conclude that increasing HIV incidence in MSM in recent years has been fuelled by a spread of HIV in high-risk sexual networks with an increasing proportion of infections transmitted during highly infective early HIV infection, acquired mostly from casual sexual partners.