Background: Global longitudinal strain (GLS) is recommended to detect subclinical changes preceding reduced left ventricular ejection fraction (LVEF) in trastuzumab related cardiotoxicity. Since the possibility to detect signs of acute myocardial deterioration at treatment initiation is not clarified, the objective of this study was to assess changes in GLS and biomarkers within the first 2 weeks of trastuzumab treatment. Methods: In a prospective cohort study, 45 patients with non-metastatic breast cancer (age 54, LVEF 62.8%, GLS -19.9%, 40% hypertension) scheduled for trastuzumab treatment were included. Echocardiography and measurement of troponin and NT-proBrain-Natriuretic-Peptide were conducted before initiation of trastuzumab, at days 3, 7, and 14 and after 3, 6, and 9 months. Results: A significant deterioration in LVEF from 62.8% (SD±3.6) to 58.4% (SD±4.1) (p < 0.0001), GLS from -19.9 (SD±2.1) to -18.1 (SD±2.5) (p = 0.004), s’ (p < 0.0001), e’ septal (p = 0.008), and s’ RV (p < 0.0001) occurred at 9 months and was preceded by significant changes in these parameters within the first 14 days. After 14 days, 12 patients (27%) had a ≥10% deterioration in GLS, which was associated with significantly lower LVEF at 55.2% (SD±4.1) at 9 months compared to patients with < 10% early deterioration in GLS (LVEF = 59.5% (SD±3.5) (p = 0.001)). No difference in plasma concentrations of biomarkers was observed between the two groups. Conclusion: In this study deteriorations in key echocardiographic parameters within normal limits were detected during the first 2 weeks of trastuzumab treatment, and an early ≥10% deterioration in GLS was associated with a lower LVEF at 9 months.