Using a precise technique for measuring authentic plasma lipid hydroperoxides (ROOHs), we show that individuals with non-insulin-dependent diabetes mellitus (NIDDM) have higher levels of ROOH than do control subjects. ROOHs were measured by the ferrous oxidation with xylenol orange assay coupled with the selective ROOH reductant triphenylphosphine. Formation of the ferric xylenol orange complex was determined at 560 nm and calibrated against H2O2. For 22 individuals with NIDDM, a concentration of 9.04 +/- 4.3 mumol/l (mean +/- SD) ROOH was recorded. This concentration was higher (P < 0.0005 by separate-variance t test) than that of plasma ROOHs from control subjects (3.76 +/- 2.48 mumol/l). There was no difference between concentrations of plasma malondialdehyde measured as thiobarbituric acid-reactive material (TBARM) in NIDDM or control subjects (1.00 +/- 0.70 vs. 1.21 +/- 0.62 mumol/l, respectively; P > 0.1). A trend to lower vitamin E levels in the NIDDM group (9.03 +/- 3.31 vs. 10.31 +/- 5.02 micrograms/ml in control subjects) failed to achieve significance at the 95% confidence level. Plasma ROOHs in the diabetic group did not correlate with total plasma cholesterol, triglyceride, fasting glucose, HbA1, vitamin E, or TBARM levels. These data indicate that measurement of authentic ROOHs shows NIDDM to be associated with oxidative stress, which may be unrelated to abnormalities in lipid metabolism and glycemic control.