OBJECTIVE:To analyse changes in the distribution of BMI in Australia between 1980 and 2000. DESIGN:Data were from the 1980, 1983 and 1989 National Heart Foundation Risk Factor Prevalence Study, the 1995 National Nutrition Survey and the 1999/2000 Australian Diabetes, Obesity and Lifestyle Study. Survey participants were aged 25-64 years and resident in Australian capital cities. BMI was calculated as weight divided by height squared (kg/m2), where weight and height were measured using standard procedures. RESULTS:Mean BMI was higher in 2000 than 1980 in all sex and age groups. The age-adjusted increase was 1.4 kg/m2 in men and 2.1 kg/m2 in women. The BMI distribution shifted rightwards for all sex and age groups and became increasingly right-skewed. The change between 1980 and 2000 ranged from a decrease of 0.04 kg/m2 at the lower end of the distribution for men aged 25-34 years to an increase of 7.4 kg/m2 at the higher end for women aged 55-64 years. While the prevalence of obesity (BMI >or= 30 kg/m2) doubled, the prevalence of obesity class III (BMI >or= 40 kg/m2) increased fourfold. CONCLUSIONS:BMI in urban Australian adults has increased and its distribution has become increasingly right-skewed. This has resulted in a large increase in the prevalence of obesity, particularly the more severe levels of obesity. It will be important to monitor changes in the different classes of obesity and the extent to which obesity interventions both shift the BMI distribution leftwards and decrease the skew of the distribution.