Background: Ado-trastuzumab emtansine (T-DM1), has improved rates of progression-free and overall survival amongst patients treated for human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (mBC). The development of spider naevi/telangiectasia represents an under-reported adverse event amongst patients being treated with T-DM1. The mechanism of development of T-DM1 associated telangiectasia is unknown. Case Report: Here, we report the case of a 53 year old female patient with metastatic HER2 positive breast cancer. Following 12 months of treatment with T-DM1, she developed many tens of spider naevi across her face, chest, and arms. She was not thrombocytopaenic, nor did her biochemistry reveal a transaminitis. She did have an elevated bilirubin, notwithstanding her history of Gilbert’s Syndrome. At the time these skin lesions appeared, she was also suffering from poly-arthralgia, presumed to be toxicity of previous anti-cancer immunotherapy. She was given a three-week course of reducing steroid, and experienced a significant reduction in the number of spider naevi. For cosmetic purposes, she has undergone a session of hyfrecation to a number of remaining lesions, and will be seen with a view to further hyfrecation in a number of months. Conclusion: The development of telangiectasia represents a newly recognized adverse effect of T-DM1, and one of which dermatologists and oncologists alike, should be aware. Oncologists should actively enquire about, and examine for, these lesions, while dermatologists should be aware that the pattern of mucocutaneous lesions can mimic hereditary haemorrhagic telangiectasia (HHT), present a bleeding risk, and produce cosmetic concerns for patients.