The age-related changes in collagen-linked fluorescence (browning) were investigated in skin from subjects with long-standing type I diabetes. Overall browning rates were 2.4 times higher in diabetics than in controls (P < 0.02) and slope intercept accurately reflected the mean age of onset of diabetes (11.6 vs. 11.2 yr), suggesting that the browning process has the attributes of a biological clock. Browning rates were not different in controls and diabetics without retinopathy (P > 0.05) but were 2.4 (P < 0.05) and 2.7 (P < 0.01) times increased in the presence of background and proliferative retinopathy, respectively. Compared with subjects with retinopathy, individual browning rates since onset of diabetes decreased with advancing age in subjects free of retinopathy (P < 0.001). Extrapolation revealed that they would become identical to that of nondiabetic subjects by the age of 66.4 yr. These results suggest the presence of a mechanism that controls the browning rate of collagen in diabetics who do not develop retinopathy.