This is a phase II, multi-center, open-label, non-randomized trial. During Part 1 of the trial, patients will receive once daily oral administration of STI571 at a dose of 600 mg for 24 weeks. After completing 24 weeks of therapy, patients may be eligible to receive additional therapy during Part 2 of the trial provided that, in the opinion of the investigator, the patient has benefited from treatment with STI571 and in the absence of safety concerns. During Part 2 (which is of indefinite duration), patients will continue to receive STI571 on a daily basis until either death, the development of intolerable toxicity or the investigator feels it is no longer in the patient's best interest to continue therapy, whichever comes first.