This is a randomized, double-blind, active-comparator (metformin) controlled study in drug-naïve patients with T2DM who have an HbA1c ≥ 7.5%. The duration of the study is 55 weeks, which will include a 1-week screening period (Visit 1 to Visit 2) and a 54 week double-blind, active treatment period. At Visit 2/Day 1 patients who meet all enrollment criteria will be randomized in a 1:1 ratio to one of two active treatment groups: 1. JANUMET™ (fixed-dose combination sitagliptin/metformin) or 2. metformin. The starting fixed-dose combination of sitagliptin/metformin will be 50/500 mg administered twice-daily and then up-titrated to a dose of 50/1000 mg b.i.d. over 4 weeks. The starting dose of metformin will be 500 mg twice-daily and then up-titrated to a dose of 1000 mg b.i.d. over 4 weeks. Patients who can not tolerate JANUMET™ at a dose of at least 50/500 mg b.i.d. or metformin at a dose of at least 500 mg b.i.d. 6 weeks after randomization and throughout the study will be discontinued. During the double-blind treatment period patients will remain on the study medication, but investigators are allowed to add other antihyperglycemic agents to improve glycemic control as necessary. The investigator can schedule additional visits to initiate additional antihyperglycemic agents or monitor glycemic control at any time during the study period. There will be no fixed visit schedule, but 7 clinical visits are recommended. It is also recommended to perform the first visit on active treatment 6 weeks after randomization and after this visit to schedule visits every 3 months. All clinical and lab data can be obtained using the "Clalit health care information system".