It has been previously demonstrated that diabetics are less sensitive to heparin compared to non-diabetics. We hypothesized that an initial heparin dose of 80 IU per kilogram administered to diabetics rather than 70 IU per kilogram might yield a more optimal initial ACT of 300 to 350 seconds when glycoprotein IIb/IIIa receptor antagonists are not used. We prospectively studied 130 elective PCI patients without diabetes treated with 70 IU per kilogram of unfractionated heparin and 81 elective PCI patients with diabetes treated with 80 IU per kilogram, and compared the initially achieved ACT. The mean heparin dose given per kg was greater (by intention) in diabetics versus non-diabetics. Despite that, there was no significant difference in the initially achieved ACT in diabetics and non-diabetics.