This 2 arm study will assess the efficacy, safety and tolerability of taspoglutide compared to placebo in obese patients with type 2 diabetes mellitus inadequately controlled with metformin monotherapy. Patients will be randomized to receive taspoglutide (10mg sc once weekly for 4 weeks followed by 20mg once weekly) or placebo sc, in addition to their prescribed, pre-existing metformin therapy.After the first 24 weeks, patients on placebo will be switched to taspoglutide 20mg once weekly (after 4 weeks on taspoglutide 10mg once weekly) The anticipated time on study treatment is 12 months, and the target sample size is 100-500 individuals.

Inclusion Criteria:

• adult patients, 18-75 years of age;
• type 2 diabetes mellitus, receiving metformin at a stable dose of >=1500mg/day for at least 12 weeks;
• HbA1c >=6.5% and <=9.5% at screening;
• BMI >=30 and <=50 kg/m2 at screening;
• stable weight +/-5% for at least 12 weeks prior to screening.

Exclusion Criteria:

• history of type 1 diabetes or acute metabolic diabetic complications such as ketoacidosis or hyperosmolar coma within the past 6 months;
• evidence of clinically significant diabetic complications;
• myocardial infarction, coronary artery bypass surgery, post-transplantation cardiomyopathy or stroke within the past 6 months.