High levels of very low density lipoprotein triglycerides and low levels of high density lipoprotein cholesterol have been found to be associated with insulin resistance measured by the euglycemic clamp technique. In contrast, the association of isolated hypercholesterolemia with insulin resistance has not been systematically studied. Therefore, we performed two separate studies designed to investigate the degree of insulin resistance in familial hypercholesterolemia (FH) (study 1) and nonfamilial hypercholesterolemia (non-FH) (study 2). Study 1 included eight young adults with FH and 13 corresponding control subjects. Fasting blood glucose, insulin, and C-peptide levels were similar in FH patients and control subjects during an oral glucose tolerance test. During the euglycemic hyperinsulinemic (1,200- 1,300 pmol/1) clamp studies, FH patients and control subjects had similar rates of whole-body glucose uptake (73±6 versus 70±3 /unol/kg per minute, respectively;p=NS). Glucose oxidation, glucose nonoxidation, lipid oxidation, suppression of free fatty acid levels, and potassium disposal were similar in both groups. Study 2 included 25 middle-aged non-FH patients and 18 corresponding control subjects. Glucose, insulin, and C-peptide responses in an oral glucose tolerance test were similar in both groups. During the euglycemic hyperglycemic clamp studies, non-FH patients and control subjects had similar rates of whole-body glucose uptake (61±3 versus 58±3 /tmol/kg per minute,p=NS). In addition, glucose oxidation, glucose nonoxidation, lipid oxidation, and suppression of free fatty acid levels as well as potassium disposal were similar in non-FH patients and control subjects. We conclude that FH and non-FH are not insulin-resistant states.