1. Male and female patients with a diagnosis of type 2 diabetes mellitus and previously treated with metformin alone, or with metformin and not more than one other oral antidiabetic drug (antidiabetic therapy has to be unchanged for 6 weeks prior to informed consent and patients should receive standard diet and exercise counseling) A dose of >/=1500 mg/day metformin is required for inclusion into the trial. The dosage needs to be stable for at least 8 weeks before randomisation. Patients with a total daily dose of less than 1500 mg metformin will only be included; if the investigator has documented them to be on their maximum tolerated dose (also in this case the 8 week time interval will apply for a stable dose).