Data on body weight and weight change collected from nationally representative samples of U.S. adults are reviewed. The body mass index (weight [kg]/height [m2]) has a low correlation with height and is used to compare body weights between persons of differing heights. The BMI varies to a greater degree in women than in men. Below the 75th percentile of the BMI distribution, women have lower BMIs than men, whereas at the 75th percentile and above, women have higher BMIs than men. Overweight is defined as a BMI of 27.8 or more in men and of 27.3 or more in women, corresponding to approximately 20% or more above desirable weight in the 1983 Metropolitan Life Insurance Company tables. For persons of average height (men, 5'9"; women, 5'4") this definition is equivalent to a body weight above 85 kg (187 pounds) in men and above 72 kg (158 pounds) in women. Among adults 20 to 74 years of age, 24% of men and 27% of women are overweight, yielding an estimated total of 34 million persons in the United States. The prevalence of overweight increases with age, for both men and women but to a greater degree in women. Blacks and Hispanics have a higher prevalence of overweight than do whites, especially among women. Between 1960 and 1980, the prevalence of overweight among whites increased by 3% in women and by 6% in men. In blacks, however, the prevalence of overweight increased by 7% in women and by 28% in men. Longitudinal body weight measurements taken 10 years apart show that adults younger than 55 years tend to gain weight, whereas those 55 years and older tend to lose weight. The youngest adults gain the most weight, and the oldest adults lose the most weight. In all age groups, women have substantially greater variation in their 10-year weight change than do men.