The HIV/AIDS epidemic has led to growing interest in the epidemiology of sexually transmitted diseases (STDs) in Switzerland. STD surveillance data from three sources are presented: reports from six policlinics of dermatovenereology since 1973, laboratory reports of Neisseria gonorrhoeae, Treponema pallidum and Chlamydia trachomatis since 1988, and reports by the Swiss Sentinel Network between June 1991 and December 1993. The data indicate that there has been a decline in the number of cases of syphilis and, in particular, gonorrhoea since the early 1980s in Switzerland. Since many factors could explain the declines (e.g. more effective treatments, more widespread use of antibiotics, marked changes in behaviour such as increased levels of condom use) it is difficult to identify their exact causes. Evidence for two of the before-mentioned factors exists and these probably played important roles in the declines. Firstly, as a result of the emergence of penicillinase producing strains of N. gonorrhoeae, the introduction of new treatments for gonorrhoea in the early 1980s which were simpler to administer and more effective (in particular the use of spectinomycin). Secondly, the national AIDS prevention campaign which began in 1987 and has been associated with major increases in condom use in the Swiss population.