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.