Inclusion Criteria:

- Age ≥ 18 years

- Unresectable and/or metastatic GIST

- Currently receiving imatinib 400 mg per day for a minimum of 4 weeks prior to
registration, and for no more than 6 months prior to registration. This must be the
first time that the patient has been treated for metastatic and/or unresectable GIST

- For patients who received imatinib following surgery at the time of an initial
diagnosis of GIST, there must be a 6 month interval between completion of imatinib
and the diagnosis of metastatic GIST

- Good physical functioning (ECOG Performance Status of 0 or 1)

- Generally, good function of organ such as liver and kidneys

Exclusion Criteria:

- Disease progression during adjuvant therapy with imatinib (adjuvant treatment is
treatment that is given after surgery for GIST)

- Known intolerance of imatinib at a dose of 400 mg/day or higher

- Prior systemic therapy for advanced GIST with imatinib or those who have been on
imatinib for longer than 6 months for unresectable and/or metastatic disease

- Major surgery within 2 weeks prior to Day 1 of study or who have not yet recovered
from prior surgery

- Use of coumadin derivatives (i.e. warfarin, acenocoumarol, phenprocoumon)

- Patients who have received wide field radiotherapy ≤ 4 weeks or limited field
radiation for palliation < 2 weeks or who have not recovered from side effects of
this therapy