Background: Targeting of HER2 with trastuzumab (T) is a well-established strategy in the metastatic and adjuvant setting in HER2 positive breast cancer (HER2-BC). Although HER2 status is routinely determined using immunohistochemistry or fluorescence in situ hybridization, technical problem can arise when lesions are poorly accessible. HER2 expression can vary during the course of the disease, and there can be discordance in HER2 expression across tumor lesion even in the same patients. Noninvasive HER2 imaging is crucial needed to solve these problems. Previous imaging using ¹¹¹In or ⁸⁹Zn- trastuzumab produced high radiation exposure to patients by their long half life (=67.9and 78.4h) and low resolution image. Half life of ⁶⁴Cu is 12.7h. We performed a feasibility study of the ⁶⁴Cu-1, 4, 7, 10- tetraazacylododecane-N,N',N",N"'-tetraacetic acid (DOTA)-T to perform PET imaging in patients with HER2-BC. Methods: Patients with HER2-BC received 150 MBq of ⁶⁴Cu-DOTA-T and underwent PET scan 1, 24 and 48h after the injection. Six patients were evaluated internal dosimetry by collecting radioactivity data of blood and normal tissue in each time point from PET study, and radiation exposure by collecting clothes, linen and urine to test feasibility for outpatients. Results: Fifteen patients who received T therapy were enrolled in the “first-in-human” trial. All patients had no severe toxicity. Radiation excretion evaluating clothes, linen were under background level. Radiation exposure of ⁶⁴Cu-DOTA-T was equivalent to that of conventional ¹⁸F-FDG-PET. Distribution of liver, kidney, spleen, and blood vessel was 2-8 SUV, and uptake in other normal tissue was low. At visual examination, in two patients brain metastases were clearly visualized by ⁶⁴Cu-DOTA-T-PET, suggesting that there are disruptions of the blood-brain barrier at the site of the brain metastases. The sternum bone metastasis was well visualized and quantitatively monitored according to the response by T. Primary breast cancers, lymph node metastases and lung metastases could also be visualized at the lesion indentified by CT. Conclusions: ⁶⁴Cu-DOTA-T-PET was feasible test even for outpatient, and provides specific and high resolution image in HER2 positive lesion.