The prevalence of sexual behaviours was estimated from a survey of 3854 Australian secondary school students. Self-reported prevalence of intercourse increased from 9.9% (age 13 years or less) to 23.9% (age 15) and 51.2% at age 17 and over. Among 932 sexually experienced youth in the final 3 years of secondary school, 26% of males and 18% of females reported having 3 or more partners in the previous year; 89.4% had used a condom at least once, with males (71.8%) reporting more last occasion condom use than females (53.4%). 'Last occasion' and 'usual', but not lifetime, condom use was significantly lower among older respondents. Although nearly 90% of females in each of 3 age groups reported lifetime condom use, just 27.6% of female students aged 17 or more reported that condoms were always used during intercourse. The decline with age, noted in research with adults and older adolescents, may begin in the middle teens.