BACKGROUND - Data are sparse regarding the association of stable body mass index (BMI) over the long term with metabolic syndrome components in young adults. METHODS AND RESULTS - Participants in the Coronary Artery Risk Development in Young Adults Study, including white and black adults 18 to 30 years of age at the initial examination in 1985 to 1986, were stratified into groups by baseline BMI and change in BMI (stable/decreased, increased >2 kg/m, or fluctuating) across all 6 examinations between years 0 and 15 of the study. Changes in metabolic syndrome components were compared between groups. Among 1358 men and 1321 women, 16.3% maintained a stable BMI, 73.9% had an increased BMI, and 9.8% had a fluctuating BMI. Over 15 years, participants with stable BMI had essentially unchanged levels of metabolic syndrome components, regardless of baseline BMI, whereas those with increased BMI had progressively worsening levels. For example, men with a baseline BMI of 20.0 to 24.9 kg/m and stable BMI during follow-up had a mean increase of only 15 mg/dL in fasting triglycerides over 15 years compared with 65 mg/dL (P<0.001) in those whose BMI increased. Incidence of metabolic syndrome at year 15 was lower in the stable BMI group (2.2%) compared with the increased BMI group (18.8%; P<0.001). CONCLUSIONS - Adverse progression of metabolic syndrome components with advancing age may not be inevitable. Young adults who maintained stable BMI over time had minimal progression of risk factors and lower incidence of metabolic syndrome regardless of baseline BMI. Greater public health efforts should be aimed at long-term weight stabilization. © 2007 American Heart Association, Inc.