Fifty patients with Philadelphia chromosome-positive (Ph(+)) chronic myelogenous leukemia (CML) in early chronic phase received imatinib mesylate, 400 mg orally daily. After a median follow-up of 9 months, 49 patients (98%) achieved a complete hematologic response and 45 patients (90%) achieved a major cytogenetic response, complete in 36 patients (72%). Compared with similar patients who received interferon-alpha with or without hydroxyurea or other interferon-alpha combination regimens, those receiving imatinib mesylate had higher incidences of complete and major (Ph < 35%) cytogenetic responses at 3 months (34% and 74% versus 1%-4% and 9%-24%, respectively), 6 months (52% and 80% versus 3%-7% and 11%-28%, respectively), and 9 months (60% and 77% versus 5%-11% and 14%-30%, respectively; P <.001). Competitive quantitative polymerase chain reaction (QPCR) studies at 9 months showed a median QPCR value (ratio of BCR-ABL/ABL transcripts x 100) of 0.59% overall and of 0.24% (range, 0.001%-29.5%) for complete cytogenetic response.