STI571 will be considered active if the interferon-refractory patient population satisfies the target of achieving a complete or major response at a rate of at least 30%, within the preset error limits. Cytogenetic responses will be evaluated every three months and categorized as either complete (0% Ph+ chromosome cells), or major (1 to 35% Ph+ chromosome cells) responses. STI571 will be discontinued for any patient whose disease progresses to either the accelerated phase or blast crisis.