The BCR-ABL fusion gene in chromosome translocation, t (9; 22), and its product, p210BCR/ABL oncogenic tyrosine kinase, is the underlying molecular mechanism that leads to the development of CML. Quantitative detection of BCR-ABL fusion gene has become a reliable approach to diagnose and monitor CML. The aim of this study was to evaluate a Roche t (9; 22) kit in CML diagnosis, monitoring treatment responses, and identification of relapse. Using BCR-ABL fusion gene-expressing K562 cells, a series of standard samples were prepared and used to establish a curve for the calculation of BCR-ABL fusion gene expression in patient samples. Our results indicate that PCR detection system with aforementioned kit has good reproducibility. In addition, the relative concentration of BCR-ABL measured by PCR was in agreement with the patient's response to the Imatinib treatment and bone marrow morphology remission. Furthermore, we found that the relative concentration of BCR-ABL fusion gene increased 1-3 months before CML relapse was clinically and cytogenetically diagnosed, suggesting that the PCR-based BCR-ABL fusion gene detection with t (9; 22) kit is able to diagnose the recurrence of CML at least 1 month earlier than the classic cytogenetic analysis. In conclusion, detection of BCR-ABL fusion gene expression in CML using Roche t (9; 22) kit has great clinical value in the primary diagnosis, monitoring treatment responses, and identification of relapse in CML patients.