BACKGROUND: The short- and long-term risks for developing overweight or obesity are unknown. OBJECTIVES: To estimate the short-term, long-term, and lifetime risks for developing overweight or obesity in adults in the community. DESIGN: Prospective cohort study, 1971 to 2001. SETTING: Community-based study, Framingham, Massachusetts. PARTICIPANTS: 4117 white participants (51.9% women) from the Framingham Heart Study. MEASUREMENTS: The short-term (4 years) and long-term (10 to 30 years) risks for ever becoming overweight or more (body mass index [BMI] > or = 25 kg/m2) or obese (BMI > or = 30 kg/m2) for men and women at 30, 40, and 50 years of age with a normal BMI (between 18.5 kg/m2 and 25.0 kg/m2). RESULTS: The observed 4-year rates of developing overweight varied from 14% to 19% in women and 26% to 30% in men. Four-year rates of developing obesity ranged from 5% to 7% in women and 7% to 9% in men. The long-term (30-year) risk estimates were similar for the 2 sexes generally; varied somewhat with age (in men, being lower for those 50 years of age); and, overall, exceeded 1 in 2 persons for overweight or more, 1 in 4 individuals for obesity, and 1 in 10 people for stage II obesity (BMI > or = 35 kg/m2) across different age groups. The 30-year estimates correspond to the residual lifetime risk for overweight or more or obesity for participants 50 years of age. LIMITATIONS: These findings may not be generalizable to other races or ethnicities. CONCLUSIONS: The long-term risks for overweight or more or obesity exceeded 50% and 25%, respectively, indicating a large public health burden. These estimates suggest that the future burden of obesity-associated diseases may be substantial.