BACKGROUND/OBJECTIVES: Low birth weight and postnatal catch-up growth have been associated with an increased risk for diabetes mellitus type II (DMII). We evaluated the contribution of birth and adult size, body composition, and waist-to-hip ratio to DMII risk factors in young adulthood. METHODS: In a group of 136 young adults, aged 18-24 yr, insulin sensitivity and disposition index were determined by frequent sampling iv glucose tolerance test. The association of clinical parameters with these variables was analyzed with multiple regression modeling. In addition, differences in insulin sensitivity and disposition index, a measure for beta-cell function, were analyzed in four subgroups, young adults either born small for gestational age SGA with short stature (n = 25) or SGA with catch-up growth (n = 23) or born appropriate for gestational age with idiopathic short stature (n = 23) or with normal stature (controls) (n = 26). RESULTS: Fat mass was the only significant predictor of insulin sensitivity, whereas birth length and birth weight were not significant. After correction for age, gender, and adult body size, insulin sensitivity was significantly lower in subjects born SGA with catch-up growth compared with controls. None of the variables had a significant influence on disposition index, and there was no significant difference in disposition index between the subgroups. CONCLUSIONS: Our data show that a higher body fat mass at 21 yr is associated with reduced insulin sensitivity, independent of birth size. These findings have important implications for public health practice.