The severity of metabolic syndrome depends on the degree of insulin resistance. However, currently there is no adequate clinical marker for quantitative analysis of insulin resistance. A small quantity of lipoprotein lipase (LPL) protein, which is an inactive form and commonly called 'preheparin LPL mass', exists in serum and is detected by a sensitive immunoassay system. Recent studies have reported the clinical significance of serum preheparin LPL mass levels in various aspects. For example, preheparin LPL mass is negatively related to serum triglyceride and positively related to HDL-cholesterol, is low in type 2 diabetes mellitus, is increased by administration of insulin sensitizer, and shows an inverse relationship with visceral adiposity. Furthermore, preheparin LPL mass level is significantly lower in patients with coronary atherosclerosis compared to patients with no lesion, and correlates negatively with the severity of these lesions. From these reports, preheparin LPL mass may be considered to be the most important quantitative indicator of insulin resistance of the whole body.