Behavioural epidemiology is an important aspect of HIV research, particularly among marginalized populations where measurement of rates of infection have not been conducted. This Canadian study provides a country-wide analysis of the characteristics and behaviours of gay and bisexual men, and examines the influence of geographic, socio-demographic and lifestyle influences on sexual behaviour and test-seeking. A purposive sample of 4,803 men was recruited through gay-identified venues. In order to provide national representation seven sampling strata were defined. Data were collected by self-completed questionnaire. A three-level hierarchical logistic regression analysis is used to model two behaviours, unprotected anal intercourse and test-seeking. The results showed that, nationally, 22.9% of respondents reported at least one episode of unprotected anal intercourse in the previous 3 months, and 63% had been tested. Characteristics and behaviours of men varied across the country. Geographic differences appear to be less important in explaining unprotected anal intercourse than test-seeking. In conclusion, policy, programmes and social environment appear to exert an important influence on test-seeking, whereas cultural and psychosocial dimensions appear to have a greater influence on sexual behaviour.