1. In this study we compared the 500 MHz 1 H-NMRs from native and oxidized low-density lipoproteins. 2. The measurements revealed a characteristic pattern of three resonances in spectra from oxidized, but not from native low-density liprotein at 1.17 p.p.m., 1.18 p.p.m. and 1.20 p.p.m. (relative to 3-trimethylsilyl-[2,2,3,3- 2 H 4 ]-propionate). 3. A quantitative comparison between these resonances in sera from patients with coronary heart disease and healthy control subjects revealed that the intensity was significantly higher in patients with coronary heart disease (1.17 p.p.m.: 0.026±0.014 versus 0.015±0.019; 1.18 p.p.m.: 0.032±0.011 versus 0.017±0.021; 1.20 p.p.m.: 0.030±0.066 versus 0.010±0.005; P < 0.05 compared with healthy control subjects for each resonance). 4. Fractionation of sera from patients with coronary heart disease revealed that the resonances equal to those obtained from experimentally oxidized low-density lipoprotein are indeed caused by the low-density lipoprotein fraction of the sera. 5. When the NMRs from sera were calibrated with oxidized low-density lipoprotein prepared by Cu 2+ oxidation, a concentration of 66.5±28.6 ; μ g/ml and 36.3±23.7 ; μ g/ml ( P < 0.05) was estimated in patients with coronary heart disease and healthy subjects respectively. Elevated levels of oxidized low-density lipoprotein also occurred in those patients with normal serum concentrations of total low-density lipoprotein. 6. The study shows a simple method to measure oxidized low-density lipoprotein in human serum and may gain interest to assess the cardiovascular risk factor profiles more completely.