Inclusion Criteria:

1. Male and female patients with a diagnosis of type 2 diabetes mellitus and previously treated with metformin alone or with metformin and one other oral antidiabetic drug
2. Stable metformin therapy of at least 1500 mg/day, or less if that is a maximum tolerated dose.
3. HbA1c at screening 6.5% to 9.0% for patients on metformin and one other antidiabetic drug, and HbA1c >7.0% to 10% for patients on metformin only
4. HbA1c >7.0% to 10.0% at Visit 2 (Start of Run-in)
5. Age >=18 and <80years
6. Body Mass Index (BMI) <=40 kg/m2
7. Signed and dated written informed consent prior to admission to the study in accordance with Good Clinical Practice (GCP) and local legislation

Exclusion Criteria:

1. Myocardial infarction, stroke or transient ischemic attack (TIA) within 6 months prior to informed consent
2. Impaired hepatic function
3. Renal insufficiency or impaired renal function
4. Diseases of the central nervous system or psychiatric disorders or clinically relevant neurological disorders that may interfere with participation in the trial
5. Chronic or clinically relevant acute infections
6. Current or chronic urogenital tract infection
7. History of clinically relevant allergy/hypersensitivity
8. Treatment with glitazones (e.g., rosiglitazone, pioglitazone), glucagon-like peptide (GLP-1) analogues, or insulin within 3 months prior to informed consent
9. Treatment with anti-obesity drugs within 3 months prior to informed consent
10. Treatment with systemic steroids or change in dosage of thyroid hormones within 6 weeks prior to informed consent
11. Alcohol abuse or drug abuse
12. Treatment with an investigational drug within 2 months prior to informed consent
13. Women of child-bearing potential who are nursing or pregnant, or who are not practicing an acceptable method of birth control, or do not plan to continue using this method throughout the study and do not agree to periodic pregnancy testing during participation in the trial