During the core phase of the study, patients will receive once daily oral administration of STI571 at a dose of 400 mg, for up to 12 months. After completing 12 months of therapy patients may be eligible to receive additional therapy provided that, in the opinion of the investigator, the patient has benefited from treatment with STI571 and in the absence of safety concerns. Patients will receive STI571 on an outpatient basis.