The purpose of this study is to demonstrate the efficacy and safety of vildagliptin 50mg qd as add-on therapy to sulfonylurea in patients with type 2 diabetes inadequately controlled with prior sulfonylurea monotherapy as compared to placebo. This study is aimed at supporting the regulatory approval in China of vildagliptin as combination therapy with sulfonylureas in the treatment of type 2 diabetes.

Inclusion Criteria:

1. Confirmed diagnosis of T2DM by standard criteria
2. Patients who have received a stable dose of SU for at least 12 weeks prior to Visit 1
3. HbA1c ≥7.5% to ≤ 11.0% at Visit 1 and Visit 3 (Week -1)
4. Age: ≥18 to ≤ 80 years at Visit 1.
5. BMI ≥ 20 and ≤ 40 kg/m^2 at visit 1.

Exclusion Criteria:

1. FPG ≥ 270mg/dl (15.0 mmol/L) at Visit 1 or Visit 3 (Week -1)
2. Severe or repetitive hypoglycemia, as defined in section 3.1, during the run-in period (between Visit 2 and Visit 4)

Other protocol-defined inclusion/exclusion criteria may apply