Open-label, multicenter study of imatinib (400mg/die p.o.)in patients with advanced gastrointestinal stromal tumors. Patients will be treated for up to 12 months. Data regarding its best clinical use in terms of tumor response, survival, tolerability and safety profile will be prospectively collected.

Inclusion Criteria:

1. Patients ≥ 18 years of age.
2. Histologically documented diagnosis of GIST, unresectable and/or metastatic and therefore incurable with any conventional multimodality approach.
3. Immunohistochemical documentation of c-kit (CD117) expression by tumor, as detected on a tumor sample taken within 6 weeks of study entry.
4. At least one measurable site of disease (RECIST Criteria), or other response assessment criteria, as appropriate.
5. Performance status 0,1, 2 or 3 (ECOG).
6. Adequate end organ function.
7. Adequate bone marrow function.
8. Female patients of child-bearing potential must have negative pregnancy test within 7 days before initiation of study drug dosing.

Exclusion Criteria:

1. Previous treatment with any other investigational agents within 28 days of first day of study drug dosing, unless the disease is rapidly progressing.
2. Other primary malignancy with < 5 years clinically assessed disease free interval, except basal cell skin cancer, cervical carcinoma in situ, or other neoplasms judged after consultation with the Steering committee to entail a low risk of relapse.
3. Grade III/IV cardiac problems as defined by the New York Heart Association Criteria.
4. Pregnancy, breast-feeding.
5. Severe and/or uncontrolled medical disease.
6. Known brain metastasis.
7. Known chronic liver disease (i.e., chronic active hepatitis, and cirrhosis).
8. Known diagnosis of human immunodeficiency virus (HIV) infection.
9. Previous treatment with chemotherapy within 4 weeks (6 weeks for nitrosoureas or Mitomycin-C).
10. Previous radiotherapy to ≥ 25 % of the bone marrow.
11. Major surgery within 2 weeks prior to study entry.