In a study population, we compared the level of malondialdehyde-modified LDL (MDA-LDL) with the concentrations of lipid parameters in serum and found a strong correlation between MDA-LDL and apolipoprotein B (apo B) concentrations. Their interrelations had a turning point at an apo B concentration of 1,150 mg/l. In diabetic patients, the ratio of MDA-LDL/apo B increased at apo B concentrations above 1,150 mg/l. This ratio represents the extent of modification of apo B by MDA. In the control subjects, this ratio remained stable. When we divided the patients into medication groups (statins and insulin), we found that the 1,150 mg/l threshold disappeared. At apo B concentrations above 1,150 mg/l, the ratio of MDA-LDL/apo B in the statin group was as high as that in the non-drug group. In the insulin group, the means of MDA-LDL/apo B in all ranges of apo B levels decreased to an extent statistically indistinguishable from those of the control group. In conclusion, insulin therapy represses LDL oxidation even at apo B concentrations > 1,150 mg/l and should be noted for its anti-oxidation properties.