Imatinib is a CYP3A4 inhibitor, while ado-trastuzumab is a CYP3A4 substrate. Imatinib can interact with ado-trastuzumab emtansine (T-DM1) and can increase T-DM1 concentrations, leading to T-DM1-related toxicity. There is no trial or case report in the literature on the concomitant use of Imatinib and T-DM1. Herein, we report a case in which T-DM1 was used effectively with imatinib in a patient with chronic myeloid leukaemia (CML) and metastatic Her-2-positive breast cancer. A 37-year female using imatinib for CML was diagnosed with breast cancer and a modified radical mastectomy was performed. Skin metastasis occurred within one year after adjuvant therapy was completed. Lung metastasis occurred after Trastuzumab + vinorelbine treatment and T-DM1 and imatinib were given to the patient. No side effects were observed except for grade 1 fatigue. This case report is the first to report the concomitant use of T-DM1 and imatinib in a patient of CML and metastatic breast cancer. Key Words: Imatinib, Ado-trastuzumab emtansine, Breast cancer, Chronic myeloid leukaemia.