Objective: Pyrotinib, a novel irreversible pan-ErbB receptor tyrosine kinase inhibitor, showed promising antitumor activity and acceptable tolerability in phase Ⅱ and phase Ⅲ randomized clinical trials. We assessed the activity and safety of oral pyrotinib for human epidermal growth factor receptor 2 (HER2) positive metastatic breast cancer patients in the real world.Methods: We retrospectively analyzed 72 HER2 positive metastatic breast cancer (MBC) patients who received oral pyrotinib based regimens at Beijing Cancer Hospital and other four hospitals (Peking University First Hospital, China-Japan Friendship Hospital, General Hospital of PLA, Peking University Third Hospital) from August 2018 to September 2019. Progression free survival (PFS), objective response rate (ORR), adverse events (AE) of pyrotinib were investigated.Results: Seventy-two patients with HER2 positive MBC were enrolled. The median age of the patients was 55 years (range: 32-79 years). Sixty-nine (95.8%) patients had received anti-HER2 treatment in the metastatic and/or (neo) adjuvant settings; 61 (84.7%) patients had received anti-HER2 treatments in the metastatic setting in terms of trastuzumab 56 (77.8%) patients, lapatinib 36 (50.0%) patients, and T-DM1 4 (5.6%) patients. Among these 72 patients who received oral pyrotinib based regimens, 62 (86.1%) patients received pyrotinib (±trastuzumab) in combination with chemotherapy, 6 (8.3%) patients received pyrotinib (±trastuzumab) in combination with endocrine therapy and 4 (5.6%) patients received pyrotinib (±trastuzumab). Sixty-five (90.3%) patients received 400 mg pyrotinib once daily as initial dose, and 7 (9.7%) patients received 320 mg. Objectiveresponse and safety to pyrotinib based therapy were evaluable in all the 72 patients. One (1.4%) patient achieved complete response (CR), 18 (25.0%) patients achieved partial response (PR), 41 (56.9%) patients had stable disease (SD), and 12 (16.7%) patients had progressive disease (PD). The ORR (CR+PR) was 26.4% and the median PFS was 7.6 months (95%CI: 5.5-9.7 months). Among the 36 patients with prior lapatinib therapy, the median PFS was 7.9 months (95%CI: 4.1-11.7 months). Among the 15 patients with brain metastasis, the median PFS was 6.0 months (95%CI: 2.2-9.8 months). The main toxicities related to pyrotinib were diarrhea in 57 (79.2%) cases, and 48 (66.7%) cases with grade 1-2 as well as 9 (12.5%) cases with grade 3.Conclusion:Pyrotinib based therapy is an effective treatment for patients with HER2 positive MBC, including patients with lapatinib treatment failure and brain metastasis, and the toxicities can be tolerated.