Abstract Background Cardiotoxicity is the main complication related to cancer therapy. Studies indicate that global longitudinal strain is an early detector of subclinical dysfunction of the left ventricle, preceding the decline in ejection fraction (EF). However, the reproducibility of such methodology has not been tested outside specialized centers. Objectives To assess the frequency of subclinical cardiotoxicity and to compare global longitudinal strain and EF measurements during the clinical course of patients undergoing chemotherapy for breast cancer. Methods This was an observational prospective study of 78 adult women who underwent serial echocardiograms (baseline and 1, 3, and 6 months after the beginning of chemotherapy), to evaluate biplane and 3D EF and global longitudinal strain. Cardiotoxicity and subclinical dysfunction were defined according to American Society of Echocardiography/European Association of Cardiovascular Imaging criteria. Statistical significance was set at p < 0.05. Results The mean age of the patients was 50.1 ± 11.48 years. The frequency of subclinical cardiotoxicity (defined by global longitudinal strain) was 14.9% after 30 days of chemotherapy, 16.7% after 3 months, and 19.7% after 6 months, compared to 4.5%, 3%, and 6.6%, respectively, when clinical cardiotoxicity was determined according to EF. The group that developed subclinical cardiotoxicity by 30 days (group A) had a higher frequency of clinical cardiotoxicity at 3 months (p=0.028) and a lower mean biplane EF after 30 days (p= 0.036) than the group that showed no evidence of subclinical cardiotoxicity (group B). Conclusion Subclinical cardiotoxicity was frequent and began early, being associated with a drop in EF during the clinical course.