Contemporary HIV diagnoses trends in Australia can be predicted by trends in unprotected anal intercourse among gay men.

Data from HIV and behavioural surveillance among gay men in Eastern Australian states in 1994-2008 showed a close relationship between trends in reported unprotected anal intercourse with casual partners (UAIC; measured in behavioural surveys) and routinely reported HIV diagnoses. Increasing UAIC was strongly associated with increasing numbers of HIV diagnoses 2 years later (P = 0.001). This suggests an important public health role of UAIC as a predictor of changes in HIV diagnoses in Australia.