A rise in BCR-ABL in patients with very low levels may not always have biologic significance. Mutation screening in the patient in early chronic phase commenced at 9 months of imatinib. The BCR-ABL level increased by 3-fold from months 24 to 26, but no mutations were detected and the patient maintained a CCR. The BCR-ABL values remained at a very low level, and the rise and fall may reflect the greater variability of the assay at low BCR-ABL levels. A rise at this level may not necessarily have biologic significance.