Oxidized substances in dietary lipids are suggested to have important roles for the development of vascular complications in patients with non-insulin-dependent diabetes mellitus (NIDDM). In experimental diabetic animals, increased absorption of oxidized lipids was reported. However, we have few evidences for their absorption in NIDDM patients. Since plant sterol, especially β-sitosterol, is not synthesized in human and used as a measure of cholesterol absorption efficacy, we examined the relation of plasma levels of β-sitosterol and lipid peroxides (LPO) in 82 subjects with NIDDM and compared those in 18 normal subjects. After admission, the patients were educated and received a weight-maintaining diet (28 kcal/kg/day) for more than 14 days. After an overnight fast, blood was taken and the plasma levels of β-sitosterol (HPLC method) and LPO were measured. Both the plasma β-sitosterol and LPO were significantly increased in NIDDM patients (β-sitosterol; control 0.29 ± 0.17 vs NIDDM 0.41 ± 0.32 mg/dl, P < 0.05, LPO; control 3.0 ± 1.7 vs NIDDM 4.0 ± 2.9, P < 0.05). Plasma LPO significantly correlated with plasma β-sitosterol (R = 0.70, p < 0.0001) in patients with NIDDM, but not in normal subjects.Furthermore, plasma levels of both β-sitosterol and LPO were correlated with glycated albumin levels (β-sitosterol; R = 0.30, P < 0.01, LPO; R = 0.35, P < 0.005) in NIDDM patients. These results suggest that altered glycemic control may induce deranged intestinal lipid absorption and atherogenic lipid profiles in NIDDM patients.