CASPERSEN, C. J., M. A. PEREIRA, and K. M. CURRAN. Changes in physical activity patterns in the United States, by sex and cross-sectional age. Med. Sci. Sports Exerc., Vol. 32, No. 9, pp. 1601–1609, 2000. To determine sex-specific, age-related changes in physical activity patterns. We examined cross-sectional data from the National Health Interview Survey, using the 1992 Youth Risk Behavior Survey supplement for adolescents and the 1991 Health Promotion/Disease Prevention supplement for adults. Physical activity patterns were modeled after Healthy People 2000 objectives. Among adolescents, physical activity patterns generally eroded most from ages 15 through 18. The “regular, vigorous activity” and strengthening patterns declined consistently from ages 12 through 21. Young adulthood (18–29 yr) often marked continuing erosion of activity patterns, whereas middle adulthood (30–64 yr) often revealed relatively stable patterns. At retirement age (65 yr), there was a stabilizing, or even an improving, tendency in activity patterns, usually followed by further erosion through the final period of life. Strengthening behavior eroded dramatically with advancing age among adults, especially among men. Among adolescents, differences between female and male respondents were large for regular, vigorous activity (11.3 percentage points greater for male respondents). In comparison with female adolescents and adults, male respondents reported much higher rates of regular, sustained activity (5.5 and 5.9 percentage points, respectively), and strengthening (18.2 and 11.3 percentage points, respectively). Among adults, levels of physical inactivity among women were moderately greater (5.5 percentage points) than for men. Absolute rates of change per year were mostly large to very large (3.0–8.0 percentage points·yr−1) during ages 15–18 yr, but, for adults, they were small (<0.5 percentage points·yr−1) for 33 of 40 sex, age, and pattern groupings. These data suggest that early and continued intervention will be necessary to offset these declines in physical activity throughout adolescence and adulthood.