To investigate whether duration of diabetes has an impact on the effectiveness of insulinization in diabetes management. This open-label, noninterventional, observational registry was conducted at >500 centers in Korea. Patients with diabetes, on oral antidiabetic drugs, with HbA1c ≥7% (53mmol/mol) in the preceding 3months, being considered for initiation of basal insulin by their physicians, were included. Data were collected at baseline and at 3 and 6months. Of 6,616 patients evaluated, 62.5% had diabetes for <10years, while only 6.5% patients had diabetes for ≥20years. At the end of study, average HbA1c in patients with diabetes for <10years, for 10 to <20years, and for ≥20years was 7.3±1.0% (56±10.9mmol/mol), 7.4±1.0% (57±10.9mmol/mol), and 7.6±1.1% (60±12.0mmol/mol), respectively. Over half the patients (50.7%) with diabetes <10years achieved HbA1c <7% (53mmol/mol) by the end of study, while only 42.1 and 35.1% patients with diabetes for 10 to <20 and ≥20years, respectively, achieved their target. The average insulin dosage required for per unit HbA1c reduction was significantly different among the groups according to duration of type 2 diabetes mellitus (p<0.05). Among patients who achieved HbA1c <7%, proportion of patients with hypoglycemia in the ≥20years group was higher than that in the <10years, 10 to <20years groups. Early insulin administration provided a better glycemic control with less insulin dosage and lower frequency of hypoglycemic events. Thus, early insulinization might hold the key to better management of type 2 diabetes mellitus.