Carbonyl stress-induced tissue damage is caused by reactive aldehydes produced by non-enzymatic glycation, oxidative stress and metabolic processes. The aim of this study was the detection of the major markers of carbonyl stress in the urine of diabetic patients (21 type 1, 77 type 2, 51 female, 47 male, 56.6 +/- 13.7 year of age; mean +/- SD). Oxidative stress was detected by using the reaction of malondialdehyde, the end product of free radical damage of the tissues, with L-arginine. This reaction produced a fluorescent compound, pyrimidinyl-L-ornithine. Thus, pyrimidinyl-L-ornithine, as well as pentosidine, an advanced glycation end product, and the non-specific advanced glycation end product, which is thought to be partially as a result of lipid peroxidation, could be detected simultaneously by using the fluorescent method. Correlation coefficients among the concentrations of these products in the urine of 98 diabetic patients were as follows: pyrimidinyl-L-ornithine vs. non-specific advanced glycation end product: r = 0.72, p < 0.001; pentosidine vs. non-specific advanced glycation end product: r = 0.68, p < 0.001; pentosidine vs. pyrimidinyl-L-ornithine: r = 0.60, p < 0.001. Strong negative correlations were found between the serum creatinine levels of these patients, between 120-240 mumol/l, and the urinary concentration of these products: r = -0.88 for non-specific advanced glycation end products, r = -0.86 for pentosidine and r = -0.89 for pyrimidinyl-L-ornithine (p < 0.001 for all three). These data support a closer relation of the so-called non-specific glycation end product to oxidative stress than to non-enzymatic glycation. Results presented here suggest an early retention of the products of carbonyl stress in the patients with moderate renal insufficiency, which can play a role in the development of diabetic complications.