This study is designed to evaluate safety and efficacy of vildagliptin versus NPH insulin add-on to glimepiride in patients with type 2 diabetes mellitus that do not reach adequate glycemic control on their current sulfonylurea monotherapy to give treating physicians a guidance which additional anti-diabetic treatment can be used if sulfonylurea monotherapy is not sufficient to reach glycemic control.

Inclusion Criteria:

• Confirmed diagnosis of type 2 diabetes mellitus.
• Contraindicated or intolerant to take metformin.
• HbA1c of ≥ 7.0% and ≤ 8.5%
• Current sulfonylurea (glimepiride) monotherapy and judged by the investigator to be inadequately controlled
• Other protocol-defined inclusion/exclusion criteria may apply

Exclusion Criteria:

• Patients who are taking any other anti-diabetes drug (oral or injection) other than an SU component in the preceding 12 weeks.
• Acute metabolic conditions such a ketoacidosis, lactic acidosis or hyperosmolar state within the past 6 month
• Patients taking sulfonylurea for longer than 5 years
• History of hypersensitivity to any of the study drugs or to drugs with similar chemical structures
• pregnancy
• Other protocol-defined inclusion/exclusion criteria may apply