Childhood adiposity and insulin resistance are well-known risk factors for adult metabolic syndrome (MetS). This study aims to examine whether the association between childhood adiposity and adult MetS is modified by insulin resistance. The cohort consisted of 1,593 black and white subjects, aged 19-50 years at follow-up, who were examined 19 years apart on average as children and adults for MetS variables. The prevalence of adult MetS was compared between the insulin-sensitive obesity and insulin-resistant obesity groups in childhood. Adult MetS prevalence was higher in the insulin-resistant obesity group than in the insulin-sensitive obesity group (34.9% vs. 24.3%, p = 0.008). In multivariable logistic regression analyses adjusted for age, race, gender, and follow-up years, individuals with insulin-resistant obesity in childhood were 1.7 times (p = 0.011) more likely to have MetS 19 years later on average than those with insulin-sensitive obesity in childhood. Odds ratio did not differ significantly between blacks and whites (p = 0.724). ORs for the association of childhood BMI with adult MetS significantly increased with increasing tertiles of childhood HOMA (p < 0.001 for trend). These findings suggest that insulin resistance amplifies the association between childhood adiposity and adult MetS and underscore the importance of preventing both adiposity and insulin resistance in early life.