Efficacy of Insulin Lispro Mix 50/50 Therapy

Patients will continue existing insulin therapy for three months. After that, the investigators will change insulin therapy according to the insulin regimen below and continue the therapy for six months. Patients will terminate from sulfonylurea treatment at the change of insulin therapy. The investigators will not change other oral hypoglycemic agents during the whole study period. Regimen: Divide the total units of all insulin per day by three and equally apply the amount to mealtime injections of insulin lispro mix 50/50 t.i.d. When the unit is indivisible and the remainder is one unit, add it to the mealtime injection of breakfast. When the remainder is two units, add each unit to mealtime injections of breakfast and dinner. Patients will terminate from the trial when their HbA1c increases by 1% and stays at the level for more than three months after the change of insulin regimen. When there is a risk of hypoglycemia at the change of insulin regimen, the investigators will divide ninety percent of the total insulin units per day by three and equally apply the amount to mealtime injections of insulin lispro 50/50. When considered to be safe, the investigators will increase the insulin unit per day to the total insulin unit at the previous treatment within two months.