The goal was to estimate the prevalence of overweight, obesity, underweight, and abdominal obesity among the adult population of Iran. A nationwide cross-sectional survey was conducted from December 2004 to February 2005. The selection was conducted by stratified probability cluster sampling through household family members in Iran. Weight, height, and waist circumference (WC) of 89,404 men and women 15 to 65 years of age (mean, 39.2 years) were measured. The criteria for underweight, normal-weight, overweight, and Class I, II, and III obesity were BMI <18.5, 18.5 to 24.9, 25 to 29.9, 30 to 34.9, 35 to 39.9, and >or=40 (kg/m(2)), respectively. Abdominal obesity was defined as WC >or=102 cm in men and >or=88 cm in women. The age-adjusted means for BMI and WC were 24.6 kg/m(2) in men and 26.5 kg/m(2) in women and 86.6 cm in men and 89.6 cm in women, respectively. The age-adjusted prevalence of overweight or obesity (BMI >or=25) was 42.8% in men and 57.0% in women; 11.1% of men and 25.2% of women were obese (BMI >or=30), while 6.3% of men and 5.2% of women were underweight. Age, low physical activity, low educational attainment, marriage, and residence in urban areas were strongly associated with obesity. Abdominal obesity was more common among women than men (54.5% vs. 12.9%) and greater with older age. Excess body weight appears to be common in Iran. More women than men present with overweight and abdominal obesity. Prevention and treatment strategies are urgently needed to address the health burden of obesity.