In the Sexually Transmitted Diseases (STD) clinic of the Amsterdam Municipal Health Service standardized data are collected about all visitors, which makes it possible to gain insight into the STD trends. In 1999, 706 new cases of gonorrhoea and 76 new cases of infectious syphilis were diagnosed, increases of 46% and 111% respectively compared with 1998. The largest increase was seen among men who had sex with men. In this group the number of diagnosed cases of anorectal gonorrhoea in 1999 was twice as high as in 1998 (186 versus 94) and the number of infectious syphilis cases four times as high (40 versus 9). These data indicate an increase in unsafe sexual behaviour, possibly because of a change in attitudes about AIDS, now that effective antiretroviral treatment is available.