The aim of this paper is to present trends in reported HIV-related behaviours among men having sex with men (MSM) in Switzerland. In 1992, 1994, 1997 and 2000 a standardized anonymous questionnaire was placed in gay newspapers and distributed in gay associations. High levels of protection (no anal sex or consistent condom use) with casual partners were found: around 90%, with a peak in 1994 and a slight decrease in 2000 to the level of 1992. With the steady partner, the level of protection was 57% in 1992, 61% in 1997 and decreased in 2000 to the same level as in 1992. The proportion of couples with both HIV statuses known increased from 49% in 1994 to 59% in 2000. In 2000, 26% of concordant HIV negative couples protected themselves compared to 83% of those exposed to an actual or potential risk (HIV status discordant or unknown). Within each category of couples, there was no change over time in the level of reported practice of anal sex and condom use. This slight decrease in prevention behaviour justifies efforts to maintain HIV/AIDS prevention targeted at gay men. [Authors]