Background: The US Food and Drug Administration (FDA) and European Medicines Agency (EMA) define a biosimilar as a biological molecule product that is highly similar to and has no clinically meaningful differences from an existing approved reference product. The trastuzumab biosimilar was the first oncology biosimilar approved in Brazil for HER2-positive breast cancer (BC) and advanced gastric cancer treatment. This study aimed to assess the effectiveness and safety of trastuzumab biosimilar (named as Zedora in Brazil) in the adjuvant setting among HER2-positive BC patients. Methods: This is a national, multicentric, observational, and prospective real-world study. Patients with early-stage HER2-positive BC who received at least one dose of trastuzumab biosimilar as adjuvant therapy were eligible. At the present date, 162 of 170 planned patients were recruited. Here, we present the preliminary safety data of the first 59 patients recruited in the study. The safety endpoint is the occurrence of adverse events (AEs), and the safety population included all patients who received at least one dose of trastuzumab biosimilar. We analyzed the AEs collected from the signature of the consent form until the end of adjuvant treatment. Results: Baseline patient characteristics among the 59 evaluated patients included a mean age of 51.7 ± 12.9 years, with 79.7% of them having a node-negative disease. Most patients had hormone-receptor-positive tumors (67.8% estrogen-receptor-positive and 55.9% progesterone-receptor-positive). The number of patients undergoing conservative surgery (67.8%) was higher than mastectomy (32.2%). The most frequent histological BC subtype was invasive ductal carcinoma (98.3%). All 59 participants received adjuvant anti-HER2 therapy, that included trastuzumab biosimilar administration trastuzumab biosimilar + pertuzumab (22.0%), and only two patients were receiving reference trastuzumab, which was switched to trastuzumab biosimilar upon entry into the study. The duration of adjuvant anti-HER2 therapy was 10.2 ± 2.2 months, ranging from 5.8 to 13.9 months. This time corresponds to, on average, 14 ± 3.2 cycles of 21 days. In total, 78.0% of the patients had at least one AE, four of them were serious AEs (6.8%), including three of severe intensity and one life-threatening. Table 1 lists the most reported AEs according to SOC (System Organ Class). Conclusions: The nature and severity of AEs observed to trastuzumab biosimilar were consistent with the known safety profile of trastuzumab. Clinical trial information: NCT03892655.