Determination of glyoxal and methylglyoxal levels in plasma is of great interest, since it allows us to evaluate oxidation processes occurring in glycated proteins. A method based on a simple derivatization procedure followed by gas chromatography/mass spectrometry (GC/MS) analysis has been developed. Ten diabetic patients were evaluated before and after improvement of glycemic control. Fasting plasma glucose, hemoglobin A1c (HbA1c), advanced glycation end products (AGE), pentosidine, glyoxal and methylglyoxal levels were measured. The percentage decreases of the levels of fasting plasma glucose, HbA1c and AGE were larger than those of pentosidine, glyoxal and methylglyoxal. These results may be explained by considering the different position of these compounds in the Maillard reaction pathways: these two sets of metabolic parameters give different pictures of patients' metabolic control. The measurement of glyoxal and methylglyoxal may be particularly important in the evaluation of the possible effect of oxidative stress. Other metabolic pathways can contribute to glyoxal production, and the observed minor decrease in these compounds can be, in principle, ascribed to such effect. However, a similar behavior of pentosidine indicates that these alternative pathways can be only partially responsible for glyoxal and methylglyoxal production.