It has been shown that patients who experience complete hematologic or at least a partial cytogenetic response to interferon will have improved survival times. In addition, evidence exists that even patients who do not demonstrate a cytogenetic response to interferon treatment can still benefit from treatment, in terms of survival, compared to patients not treated with interferon. This indicates that if a patient is better able to tolerate interferon, he or she may have improved survival even without cytogenetic response. Preliminary studies suggest that PEG-Intron is more convenient for patients (administered once weekly rather than daily), is better tolerated than interferon, and can produce hematologic remission in interferon-a resistant patients. Phase II studies are needed to ascertain the overall hematologic and cytogenetic response rates to PEG-Intron in such patients.