Insulin sensitivity and insulin secretion are traits that are both genetically and environmentally determined. The aim of this study was to describe the distribution of the insulin sensitivity index (Si), the acute insulin response, and glucose effectiveness (Sg) in young healthy Caucasians and to estimate the relative impact of anthropometric and environmental determinants on these variables. The material included 380 unrelated Caucasian subjects (18-32 yr) with measurement of Si, Sg and insulin secretion during a combined intravenous glucose (0.3 grams/kg body weight) and tolbutamide (3 mg/kg body weight) tolerance test. The distributions of Si and acute insulin response were skewed to the right, whereas the distribution of Sg was Gaussian distributed. Sg was 15% higher in women compared with men (P < 0.001). Waist circumference, body mass index, maximal aerobic capacity, and women's use of oral contraceptives were the most important determinants of Si. Approximately one-third of the variation of Si could be explained by these factors. Compared with individuals in the upper four-fifths of the distribution of Si, subjects with Si in the lowest fifth had higher waist circumference, higher blood pressure, lower VO2max, and lower glucose tolerance and fasting dyslipidemia and dysfibrinolysis. Only 10% of the variation in acute insulin response could be explained by measured determinants. Estimates of body fat, maximal aerobic capacity, and women's use of oral contraceptives explain about one-third of the variation in Si in a population-based sample of young healthy Caucasians.