Patients With Type 2 Diabetes Mellitus With Insufficient Glycaemic Control Despite Treatment With Metformin Alone or Metformin in Combination With a Sulfonylurea

Inclusion Criteria: * Diagnosis of type 2 diabetes mellitus prior to informed consent * Male and female patients on a diet and exercise regimen who are pre-treated with immediate release metformin or immediate release metformin plus sulfonylurea (see below for minimum doses). The treatment regimen has to be unchanged for 12 weeks prior to randomisation. 1. Minimum dose for metformin: * ≥1500 mg/day or * maximum tolerated dose or * maximum dose according to local label 2. Minimum dose for sulfonylurea: * ≥half of the maximal recommended dose or * maximum tolerated dose or * maximum dose according to local label * HbA1c of ≥ 7.0% and ≤ 10% at Visit 1 (screening) in order to be eligible for randomized treatment. HbA1c of \> 10% at Visit 1 (screening) in order to be eligible for the open-label treatment arm (25 mg BI 10773) * Age ≥ 18 * BMI ≤ 45 kg/m2 (BodyMass Index) at Visit 1 (Screening) * Signed and dated written informed consent by date of Visit 1 in accordance with GCP and local legislation Exclusion Criteria: * 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) * Any other antidiabetic drug within 12 weeks prior to randomisation except those mentioned in inclusion criterion 2 * Acute coronary syndrome including myocardial infarction, stroke or TIA within 3 months prior to informed consent * Indication of liver disease, defined by serum levels of either ALT (SGPT), AST (SGOT),or alkaline phosphatase above 3 x upper limit of normal (ULN) as determined during screening and/or run-in phase * Impaired renal function, defined as eGFR\<30 ml/min (severe renal impairment) as determined during screening and/or run-in phase * Bariatric surgery within the past two years and other gastrointestinal surgeries that induce chronic malabsorption * Medical history of cancer (except for basal cell carcinoma) and/or treatment for cancer within the last 5 years * Contraindications to metformin and/or sulfonylurea according to the local label for those patients that enter the study with the respective background therapy * Blood dyscrasias or any disorders causing haemolysis or unstable Red Blood Cell (e.g.malaria, babesiosis, haemolytic anaemia) * 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 * 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 * Pre-menopausal women (last menstruation ≤ 1 year prior to informed consent) who: * are nursing or pregnant or * are of child-bearing potential and are not practicing an acceptable method of birthcontrol, or do not plan to continue using this method throughout the study and do not agree to submit to periodic pregnancy testing during participation in the trial. Acceptable methods of birth control include tubal ligation, transdermal patch, intra uterine devices/systems (IUDs/IUSs), oral, implantable or injectable contraceptives, sexual abstinence (if acceptable by local authorities), double barrier method and vasectomised partner * Alcohol or drug abuse within the 3 months prior to informed consent that would interfere with trial participation or any ongoing condition leading to a decreased compliance to study procedures or study drug intake * Participation in another trial with an investigational drug within 30 days prior to informed consent * Any other clinical condition that would jeopardize patients safety while participating in this clinical trial