This is a multicenter, post-marketing, clinical study evaluating the safety and efficacy of postoperative adjuvant therapy with Imatinib Mesylate in high-risk patients after curative resection of newly diagnosed GIST.Patients will be evaluated for relapse -free survival as measured by the confirmation of tumor recurrence and survival for 3 years after surgery for their primary tumors.

Inclusion Criteria:

• underwent macroscopically curative resection;
• immunohistochemically confirmed KIT (CD117)-positive tumors;
• judged as being high-risk according to the criteria for risk classification

Exclusion Criteria:

• synchronous double cancer or metachronous double cancer with a disease-free period of ≤5 years;
• received therapy with Imatinib Mesylate prior to study entry;
• cardiac problem of Grade 3 or higher (New York Heart Association (NYHA) criteria)

Other protocol-defined inclusion / exclusion criteria may apply.