Lipoprotein lipase mass exists in preheparin serum, even though the activity is scarcely found. The implication of this is unclear. We studied the effect of an insulin sensitizer, troglitazone, on this preheparin serum lipoprotein lipase mass (preheparin LPL mass) in non-insulin-dependent diabetes mellitus (NIDDM) patients as well as on serum lipid levels and low density lipoproteins (LDL) particle size. Thirty-one NIDDM patients with poor control were administered troglitazone 400 mg/day. Hemoglobin A1c had significantly decreased (13%, P<0.001) 2 months later. Preheparin LPL mass had gradually increased and a 69% increase (P<0.01) was observed 4 months later. Triglyceride significantly decreased (23%, P<0.01) and high density lipoprotein-cholesterol increased (10%, P<0.0l), whereas total cholesterol and LDL levels did not change 4 months later. The size of LDL increased significantly (P<0.01). These results were consistent with the idea that preheparin LPL mass might be relating to the insulin sensitivity enhanced by troglitazone, as well as LDL particle size.