Cross-sectional data, for example, suggest that weight gain is pronounced during the late teens through age 30, particularly in males, and that a linear relationship exists between weight and lipoprotein fractions at these ages. The reasons for and consequences of this increase in adiposity need further investigation. The interaction of life events, behavior, and changes in physical activity and dietary intake that may influence weight gain and lipoprotein levels should be determined, as well as the importance of weight gain in relationship to risk factor changes during this age span.