OBJECTIVE: To measure the prevalence of obesity in Australian adults and to examine the associations of obesity with socioeconomic and lifestyle factors. DESIGN: AusDiab, a cross-sectional study conducted between May 1999 and December 2000, involved participants from 42 randomly selected districts throughout Australia. PARTICIPANTS: Of 20,347 eligible people aged > or = 25 years who completed a household interview, 11,247 attended the physical examination at local survey sites (response rate, 55%). MAIN OUTCOME MEASURES: Overweight and obesity defined by body mass index (BMI; kg/m(2)) and waist circumference (cm); sociodemographic factors (including smoking, physical activity and television viewing time). RESULTS: The prevalence of overweight and obesity (BMI > or = 25.0 kg/m(2); waist circumference > 80.0 cm [women] or > or = 94.0 cm [men]) in both sexes was almost 60%, defined by either BMI or waist circumference. The prevalence of obesity was 2.5 times higher than in 1980. Using waist circumference, the prevalence of obesity was higher in women than men (34.1% v 26.8%; P < 0.01). Lower educational status, higher television viewing time and lower physical activity time were each strongly associated with obesity, with television viewing time showing a stronger relationship than physical activity time. CONCLUSIONS: The prevalence of obesity in Australia has more than doubled in the past 20 years. Strong positive associations between obesity and each of television viewing time and lower physical activity time confirm the influence of sedentary lifestyles on obesity, and underline the potential benefits of reducing sedentary behaviour, as well as increasing physical activity, to curb the obesity epidemic.