The effects of metabolic control on both antithrombin III (AT III) activity and AT III plasma concentration in 20 insulin-treated diabetic subjects have been evaluated. Basal AT III activity was significantly lower in diabetic subjects versus healthy controls (P < 0.001), whereas no difference was found in AT III concentration. A good correlation was found between AT III activity and AT III concentration (r = 0.81; P < 0.001) in healthy controls, but this correlation was not significant in diabetic subjects (r = 0.12; P = NS). In those subjects a linear inverse correlation was found to exist between AT III activity and level of glycosylated proteins (r = −0.43; P < 0.05). Diabetic subjects were also examined after 1 and 2 mo of restored metabolic control, obtained by human insulin (DNA-recombinant) therapy. Improved metabolic control was characterized by an increase of AT III activity (P < 0.05), a decrease of mean daily blood glucose, and stable HbA1 and glycosylated proteins (P < 0.05), while AT III concentration did not vary. On the other hand, a significant inverse correlation between AT III activity and glycosylated proteins was found during both the first and second months (r = −0.54 and r = −0.53, respectively; P < 0.01). Moreover, no correlation between AT III activity and AT III concentration was found. These data suggest that impaired metabolic control may alter the biologic activity of AT III in diabetes, but not its plasma concentration.