OBJECTIVE To assess whether circulating insulin is a major contributor to adverse lipid profiles during the transition from adolescence to young adulthood. METHODS The association between fasting insulin levels and serum lipid and lipoprotein levels was examined in a cross-sectional survey of 4136 young individuals aged 5 to 30 years from a biracial community. RESULTS Fasting insulin levels were strongly and positively correlated with serum triglyceride and very-low-density lipoprotein cholesterol levels and negatively correlated with high-density lipoprotein cholesterol levels in all age groups (5 to 11, 12 to 17, 19 to 24, and 25 to 30 years). An increasing impact of insulin level on low-density lipoprotein cholesterol level was observed in young adults aged 25 to 30 years. In multivariate analysis, fasting insulin level was associated with very-low-density lipoprotein cholesterol level for most of the age groups in both races independently of age, sex, glucose levels, obesity, cigarette smoking, and alcohol intake. The independent relationship to low-density lipoprotein cholesterol level persisted in young adults aged 25 to 30 years. The independent and negative association with high-density lipoprotein cholesterol level remained in whites aged 5 to 24 years and blacks aged 19 to 24 years. When individuals were divided into tertiles according to insulin concentration and subscapular skinfold thickness, the independent effect of insulin level and obesity on lipoprotein fractions was also noted. Furthermore, a stronger association of insulin level with lipoprotein fractions was observed in obese than in lean white males. CONCLUSIONS These data indicate that an increasing association of insulin levels with adverse lipoprotein levels in young adults, especially obese individuals, may have adverse consequence for adult cardiovascular diseases.