We addressed trastuzumab emtansine (T-DM1) e cacy in HER2+ metastatic breast cancer patients treated in real-world practice, and its activity in pertuzumab- pretreated patients. We conducted a retrospective, observational study involving 23 cancer centres, and 250 patients. Survival data were analyzed by Kaplan Meier curves and log rank test. Factors testing signi cant in univariate analysis were tested in multivariate models. Median follow-up was 15 months and median T-DM1 treatment-length 4 months. Response rate was 41.6%, clinical bene t 60.9%. Median progression-free and median overall survival were 6 and 20 months, respectively. Overall, no di erences emerged by pertuzumab pretreatment, with median progression-free and median overall survival of 4 and 17 months in pertuzumab- pretreated (p=0.13), and 6 and 22 months in pertuzumab-naïve patients (p=0.27). Patients who received second-line T-DM1 had median progression-free and median overall survival of 3 and 12 months (p=0.0001) if pertuzumab-pretreated, and 8 and 26 months if pertuzumab-naïve (p=0.06). In contrast, in third-line and beyond, median progression-free and median overall survival were 16 and 18 months in pertuzumab- pretreated (p=0.05) and 6 and 17 months in pertuzumab-naïve patients (p=0.30). In multivariate analysis, lower ECOG performance status was associated with progression-free survival bene t (p<0.0001), while overall survival was positively a ected by lower ECOG PS (p<0.0001), absence of brain metastases (p 0.05), and clinical bene t (p<0.0001). Our results are comparable with those from randomized trials. Further studies are warranted to con rm and interpret our data on apparently lower T-DM1 e cacy when given as second-line treatment after pertuzumab, and on the optimal sequence order.