Background National trends in sexual behaviour have been assessed mainly in the context of the HIV related behavioural surveillance system set up in Switzerland between 1987 and 1992.

Methods Several populations are included in the system. Repeated surveys have been regularly conducted among the general population and youth, men having sex with other men (MSM), injecting drug users (IDU). Data on sexual behaviour are regularly recorded among people living with HIV/Aids (PLWHA) included in the Swiss HIV Cohort.

Results The main trends observed are :

In young adults (aged 18–20):

• a steady increase in the proportion of sexually active at age 17

• a stable median number of partners with a recent increase in the proportion of multipartners;

• a high and stable level of condom use among multipartners.

Among MSM:

• an increase in the number of partners and a steady increase in unprotected anal intercourse since 1997, after a period of decreasing trends.

Among IDU:

• a low and stable use of condoms with stable partners;

• a high and stable use of condoms with occasional and paying partners (only among women) with a possible recent decreasing trend.

Among PLWHA: a high use of condoms with all types of partners with a recent decrease.

Conclusions The behavioural surveillance system in place allowed to assess various trends in sexual behaviour in several populations such as: long term trends regarding sexual debut, stable trends and recent changes regarding different indicators of sexual activity in the general population, IDU and PLWHA, inversion of trends in sexual activity and condom use among MSM.