Pentosidine is an advanced glycosylation end product and protein cross-link that results from the reaction of pentoses with proteins. Recent data indicate that long-term glycation of proteins with glucose also leads to pentosidine formation through sugar fragmentation. In this study, the relationship between the severity of diabetic complications and pentosidine formation was investigated in collagen from skin-punch biopsies from 25 nondiabetic control subjects and 41 IDDM patients with diabetes duration >17 yr. Pentosidine was significantly elevated in all IDDM patients versus control subjects (P < 0.0001). It correlated strongly with age (P < 0.0001) and weakly with duration (P < 0.082). Age-adjusted pentosidine levels were highest in grade 2 (severe) versus grade 1 and 0 complication in all four parameters tested (retinopathy, proteinuria, arterial stiffness, and joint stiffness). Significant differences were found for retinopathy (P < 0.014) and joint stiffness (P < 0.041). The highest degree of association was with the cumulative grade of individual complication (P < 0.005), determined by summing indexes of all four parameters. Pentosidine also was significantly elevated in the serum of IDDM patients compared with control subjects (P < 0.0001), but levels were not significantly correlated with age, diabetes duration, complication, or skin collagen pentosidine (P > 0.05). A high correlation between pentosidine levels and long-wave collagen-linked fluorescence also was observed, suggesting that pentosidine is a generalized marker of accelerated tissue modification by the advanced glycosylation/Maillard reaction, which is enhanced in IDDM patients with severe complications.