This study sought to determine the prevalence of American Heart Association/American College of Cardiology Foundation (AHA/ACCF) heart failure (HF) stages after potentially cardiotoxic chemotherapy was initiated. For individuals receiving potentially cardiotoxic chemotherapy, the frequency of transitioning from Stage A to more advanced HF stages is not well described. In 143 Stage A HF patients with breast cancer, lymphoma and leukemia, renal cell carcinoma, or sarcoma prior to and then at 3, 6, and 12 to 24 months after potentially cardiotoxic chemotherapy was initiated, we obtained blinded cardiac magnetic resonance measurements of left ventricular ejection fraction (LVEF). Three months after potentially cardiotoxic chemotherapy was initiated, 18.9% of patients transitioned from Stage A to Stage B HF. A total of 83% and 80% of patients with Stage A HF at 3 months, respectively, exhibited Stage A HF at 6 and 12 to 24 months; 68% and 56% of those with Stage B HF at 3 months, respectively, exhibited Stage B HF at 6 and 12 to 24 months (p < 0.0001 and p = 0.026, respectively). Transitioning from Stage A to Stage B or remaining in Stage A HF 3 months after potentially cardiotoxic chemotherapy was initiated relates to longer-term (6 to 24 months post-treatment) assessments of HF stage. Display omitted