This is a cross-sectional study conducted in Port Moresby and 3 rural areas of Papua New Guinea from 1999 to 2002. These areas were selected because of their specific characteristics such as modernity, geographical location and remoteness. The aim of the study was to compare the body mass index (BMI) of selected urban and rural populations. When age was standardized, in urban and periurban populations, the mean BMI increased with age to about 40 years, plateaued and then decreased in older age. The BMI was higher in Port Moresby than in the other study areas: many people in Port Moresby were overweight (40%) and obese (21%), and by gender, 26% of females and 16% of males were obese. In Manus, the prevalence of overweight and obesity was 36% and 18% respectively. In both Port Moresby and Manus, more women than men were obese. Obesity was not a problem in rural areas of Strickland and Central Province. In rural Central Province 52% of subjects had a BMI < 20 kg/m2. Obesity is becoming a public health problem in the urban areas. The high prevalence of overweight and obesity corresponds with the high intake of refined carbohydrates and fatty foods in urban and periurban areas. It will be necessary to carry out health awareness and education on the risk factors associated with obesity in the urban and periurban areas and promote healthy environments: healthy foods should be available and affordable, and the accessibility and safety of exercise and walking tracks must be supported by the community and government agencies.