Introduction Breast cancer is the most common cancer in women. Human epidermal growth factor receptor 2 (HER2) positivity rate is 20% and generally has a poor prognosis. Ado-trastuzumab emtansine (T-DM1) is an antibody-drug conjugate consisting of HER2 target monoclonal antibody trastuzumab and microtubule inhibitor emtansine. The most common side effects are fatigue, diarrhea, anemia, and it is generally a safe and tolerable agent. Case report In our case, we reported our patient who developed mucosal and cutaneous telangiectasia after T-DM1 treatment and who had a complete response in metastases after skin lesions. Management and Outcome While no side effects were observed during the use of T-DM1 for HER2 positive disease, nose bleeding and spider telangiectasia on the skin developed in the 9th month of the treatment. In these lesions, which did not require any treatment, no regression was observed during T-DM1 treatment. Discussion We think that T-DM1, which was detected with a low incidence of skin toxicity in studies, may form telangiectatic lesions due to vascular dilatation through emtansine, and therefore care should be taken in the treatment of T-DM1.