The prevalence of obesity in some lower-income and transitional countries is as high as, or even higher than, the prevalence reported in developed nations, and it seems to be increasing rapidly. In most countries, the prevalence of obesity is higher in women than in men, and higher in urban than in rural areas. Preobesity prevalence is very high in most Latin-American countries. Sixty per cent of the population in Venado Tuerto (Argentina) has a body mass index (BMI) of ≥25 kg m−2, as do 35% of the population in Brazil, 60% in Mexico, 68% in Paraguay and 53% in Peru. Trends are available from Brazil, where marked increases in the prevalence of obesity have occurred, except in women from higher-income groups. Women from the higher-income quartiles in urban regions experienced a marked reduction in obesity prevalence from 1989 to 1997 (12.8 to 9.2%). Although data in children is scant, the prevalence of undernutrition is decreasing and the prevalence of obesity is high also in Latin-American children. The prevalence of obesity is high even in minority Indian groups. Rapid changes in dietary structure (in particular associated with urbanization) and major changes in the levels of physical activity, both occupationally and during leisure time, may explain these changes.