The objective of this study was to determine the effects of age and life-style factors on body mass index (BMI) in a longitudinal, community-based sample. A total of 568 men and 668 women (20-60 years of age) were randomly chosen from four Northern California communities and followed for up to 7 years. Age, sex, marital status, smoking status, hours of television watched, frequency of consumption of several food items, and physical activity were used to predict rate of change of body mass index (BMI-slope). BMI increased the most for both sexes through at least age 54. The BMI-slope was higher for women compared with men, and for smokers who stopped compared with those who never smoked or continued to smoke during the study. The BMI-slopes were lower for individuals who increased activity. Other life-style variables had weak or inconsistent effects on the BMI-slope. We conclude that the BMI-slope increases over age for both sexes and that increased physical activity may reduce the BMI-slope.