Surveillance systems have been developed in Victoria to determine trends in sexually transmissible diseases (STDs). Notifications to the Health Department (including laboratory notification since May 1990) have been supplemented by data about strains of Neisseria gonorrhoeae and cultures for Chlamydia trachomatis processed by the Microbiological Diagnostic Unit, enhanced laboratory surveillance of syphilis, and data on genital herpes and genital warts from the Melbourne Sexual Health Centre. During the period under review the incidence of gonorrhoea declined, rapidly at first, and then more slowly. For women, this trend has continued, while gonorrhoea acquired abroad by men has become relatively more important. Since 1988, gonorrhoea in homosexual men has increased, and rectal isolates have increased concurrently, raising concerns about HIV risk behaviour. Cases of syphilis are likely to be ascertained through STD, antenatal and refugee screening, rather than because of symptoms or contact tracing. Chlamydia is a the most common notifiable STD, despite underreporting and underdiagnosis. In 1991, 832 cases were notified, increasing to 1377 in 1992. In 1992, of the 73 cases (65 per cent of notifications) where the doctor identified a risk, 15 per cent was attributed to homosexual contact, and 27.4 per cent to heterosexual exposure. Limitations in the data include inadequate standard case definitions for many STDs, changes in the statutory requirement for notifications in 1990, underreporting, changes in diagnostic and screening patterns, and lack of detailed demographic data. Education of general practitioners is needed to improve diagnosis and notification of chlamydia.