This study will investigate the efficacy and safety of BI 10773 in type 2 diabetic patients in order to provide these data for approval for BI 10773 by regulatory authorities as an antidiabetic agent as add-on therapy to pioglitazone alone or in combination with metformin.

Inclusion criteria:

1. Diagnosis of type 2 diabetes mellitus prior to informed consent.
2. Male and female patients on diet and exercise regimen who are pre-treated with pioglitazone alone or in combination with metformin. The treatment regimen should be unchanged for 12 weeks prior to randomisation.
3. HbA1c of >/= 7.0% and </= 10.0% at Visit 1 (screening).
4. Age >/= 18.
5. BMI </= 45 kg/m2 (Body Mass Index) at Visit 1 (screening).
6. Signed and dated written informed consent by date of Visit 1 in accordance with Good Clinical Practice (GCP) and local legislation.

Exclusion criteria:

1. Uncontrolled hyperglycaemia with a glucose level > 240 mg/dl (> 13.3 mmol/l) after an overnight fast during placebo run-in and confirmed by a second measurement (not on the same day).
2. Any other antidiabetic medication within 12 weeks prior to randomisation, except those defined as the permitted background therapy via inclusion criteria no. 2.
3. Myocardial infarction, stroke or transient ischaemic attack (TIA) within 3 months prior to informed consent.
4. Indication of liver disease, defined by serum levels of either alanine transaminase (ALT/SGPT), aspartate transaminase (AST/SGOT), or alkaline phosphatase above 3 x upper limit of normal (ULN) as determined during screening or during the placebo run-in period (i.e. at a visit prior to the randomisation visit, Visit 3).
5. Impaired renal function, defined as eGFR (estimated Glomerular Filtration Rate) < 30 ml/min (severe renal impairment, MDRD [Modification of Diet in Renal Disease] formula) as determined during screening or during the placebo run-in period (i.e. at a visit prior to the randomisation visit, Visit 3).
6. Bariatric surgery within the past two years and other gastrointestinal surgeries that induce chronic malabsorption.
7. Medical history of cancer (except for basal cell carcinoma) and/or treatment for cancer within the last 5 years .
8. Blood dyscrasias or any disorders causing haemolysis or unstable red blood cells (e.g. malaria, babesiosis, haemolytic anaemia).
9. Contraindications to pioglitazone according to the local label.
10. Contraindication to pioglitazone and/or metformin (relevant only for those patients who enter the study with both these background therapies) according to the local labels.
11. Treatment with anti-obesity drugs (e.g. sibutramine, orlistat) 3 months prior to informed consent or any other treatment at the time of screening (i.e. surgery, aggressive diet regimen etc.) leading to unstable body weight.
12. Current treatment with systemic steroids at time of informed consent or change in dosage of thyroid hormones within 6 weeks prior to informed consent or any other uncontrolled endocrine disorder except T2D.
13. Pre-menopausal women (last menstruation \