The accuracy of patients and health care professionals in drawing up the components of a split-mix insulin regimen and of premixed insulin at three dosages (30 units, 10 units, and 5 units) was assessed with both 30-U and 100-U syringes. Both patients and professionals were inaccurate with both syringes. At low doses tested and with both syringes, patients and professionals showed greater accuracy with premixed insulin than with self-mixed insulin. Errors in drawing up insulin involved errors mainly in the ratio rather than the total volume of insulin drawn. Accuracy improved with higher doses of insulin. The patients' perception of their skill level in drawing up and mixing insulin correlated with the objective measurements of their skill. The improved accuracy in dosing with premixed insulin might explain the previously observed improved glycemic control in patients taking premixed insulin.