The use of imatinib for the treatment of chronic myeloid leukemia has significantly improved the survival of most patients with this disease. This drug works by directly inhibiting the cancer-causing protein BCR-ABL that is found inside the leukemia cells and therefore is the first successful targeted treatment for cancer. Measuring the amount of the gene product for BCR-ABL by very sensitive laboratory techniques as it decreases during therapy has been extremely helpful in monitoring disease activity. Furthermore, in a subgroup of patients whose leukemia cells decrease by more than a thousand-fold measured by this sensitive technique during the first one or two years, these patients have a greater than 97% chance that disease remains under excellent control for more than five years. Therefore, discovering tests that would predict this marked decrease in BCR-ABL gene product at early timepoints when any treatment changes could be done would be very useful. To study this, a test that measures the amount of imatinib in a patient's bloodstream plasma has been recently developed at a national reference laboratory, Warnex Medical Laboratories in Laval, Quebec. Preliminary results from other national laboratories indicates that patients differ in how much imatinib their bodies absorb and get into their bloodstream, and that patients who absorb imatinib less well have a slightly lower chance of this thousand-fold reduction in leukemia levels. However, because imatinib must get inside leukemia cells to inhibit BCR-ABL, it likely that the amount of imatinib that is inside leukemia cells would predict even better the response to the drug. Warnex Laboratories has recently developed a test that can measure imatinib levels inside cells, and preliminary results indicate that patients differ in how much imatinib gets inside their cells. Dr. Suzanne Kamel-Reid's laboratory at Toronto General Hospital has also developed a test to measure the amount of gene product for the pore that imatinib uses to enter leukemia and other blood cells (called h-OCT-1), and her preliminary results also show that differs among patients. It is not clear if these differences are due to differences in the leukemia cels themselves between patients, or in differences between the way imatinib would get into any blood cells between patients. Therefore we are undertaking this current study to see (1) which of the tests, or combination of tests (bloodstream imatinib level, imatinib level inside cells, or h-OCT-1 level) best predicts how well patients patients respond to imatinib by the sensitive test for BCR-ABL, and (2) whether the differences between patients in the level inside cells is due specifically to the leukemia cells. In this trial, we will measure these levels on patients who are about to begin treatment for their chronic myeloid leukemia with imatinib. For practical purposes, participation in this trial would require patients to have extra blood samples drawn at several visits when they would normally see their hematologist/oncologist to monitor their disease and its treatment: within 28 days up to and including Day 1 as long as collected prior to start of treatment with Imatinib, and once Imatinib treatment is started, at 2 weeks, one month, 6 months , and 12 months. By obtaining this information, we may better understand how to adjust imatinib therapy to achieve the best response for more patients in the future.