The Maillard protein cross-link pentosidine is a fluorescent condensation product of lysine, arginine and ribose. It accumulates in human tissues with age, and the accumulation process is accelerated in the tissues of diabetic patients. Using SP-Sephadex C-25 in the pretreatment for HPLC, we examined levels of pentosidine in urine without hydrolysis (free form) and levels of pentosidine in urine after hydrolysis (total forms), from 23 diabetic patients and 21 control subjects. The mean percentages of the values of free form per total forms (+/- SD) were 89 +/- 15% in diabetic patients, 88 +/- 16% in control subjects and 89 +/- 15% in total populations of diabetic patients and control subjects. There was a significant correlation between the values of free form and total forms in diabetic patients (r = 0.983, p = 0.0001), in control subjects (r = 0.820, p < 0.02) and in total populations of diabetic patients and control subjects (r = 0.951, p = 0.0001). The mean level of pentosidine per mol creatinine (+/- SD) was significantly elevated in urine from diabetic patients as compared to the level in control subjects (8.8 +/- 4.3 mumol/mol creatinine vs 4.2 +/- 1.4 mumol/mol creatinine, p = 0.0001 in free form; 10.1 +/- 5.3 mumol/mol creatinine vs 4.7 +/- 1.4 mumol/mol creatinine, p = 0.0001 in total forms). These results demonstrate that urinary pentosidine, especially in free form, could be a useful marker for the assessment of diabetes and diabetic complications.