ARM I: Patients receive oral imatinib mesylate (Gleevec; STI571) once daily. Treatment continues for 1 year in the absence of unacceptable toxicity. Patients who develop a recurrence during the year of initial treatment receive imatinib mesylate (Gleevec; STI571) at an increased dose. Patients who develop a recurrence after the year of initial treatment restart imatinib mesylate (Gleevec; STI571) and continue taking the drug at the discretion of the principal investigator.