Eighteen obese inpatients with insulin resistance revealed by i.v. insulin test and expressed in various grades of hyperinsulinemia and hyperglycemia were examined for plasma lipid levels. A significant positive correlation was found to be present between the plasma triglyceride (TG) level and the insulin response to glucose load. A stepwise multiple regression analysis revealed that the insulin secretory response, the plasma cholesterol level and the relative body weight contributed to the level of plasma TG. No difference was found in the grades of insulin resistance between patients with and without elevated TG. The ratio of sum of plasma insulin values to that of blood glucose values during glucose tolerance test was markedly increased in patients with elevated TG. The patients with relatively blunted insulin response and impaired glucose tolerance curves showed only slight slight hypertriglyceridemia. Endogenous hypertriglyceridemia in obesity seems to be more closely correlated with plasma insulin level, and therefore, with insulin action rather than insulin resistance.