We investigated the influence of insulin treatment on the patients' QOL and relationship of the QOL to diabetes treatment-related clinical parameters using the newly developed “Insulin Therapy Related QOL questionnaire at night” (ITR-QOLN). Type 1 diabetes patients showed lower QOL scores than that in type 2 diabetes patients. With regard to glycemic control, patients with lower HbA₁c or lower mean fasting plasma glucose values before breakfast showed higher QOL scores. Further, higher QOL scores were seen in patients with lesser variations of the plasma glucose before breakfast or at bedtime. Although the QOL scores were low in patients with episodes of hypoglycemia, the scores were higher in patients who were confident of treating the hypoglycemia episodes by themselves. In conclusion, it was demonstrated that both achieving desirable blood glucose control and aiming at fewer hypoglycemia episodes are essential to achieve higher QOL in patients receiving insulin treatment.