IntroductionLetrozole is an aromatase inhibitor (AI), which suppresses plasma estrogen levels by inhibiting the aromatase enzyme, that causes the conversion of androgens to estrogen in peripheral tissues. There is very few information in the literature regarding the development of acute heart failure after AI use. We would like to report a case of a patient who was started letrozole for hormonal treatment and developed acute heart failure due to this.Case reportFifty-seven-year-old woman with hormone positive breast cancer was operated after neoadjuvant chemotherapy and received radiotherapy. Letrozole treatment was initiated after radiotherapy and the patient presented to the emergency department after two weeks with cough and dyspnea. Ejection fraction decreased to 20% and acute heart failure developed.Management and outcomeLetrozole treatment was stopped. Afterwards, improvement in the patient's cardiac functions was observed. Letrozole-induced heart failure was thought to be reversible.DiscussionLetrozole is one of the most commonly used HT in the treatment of hormone positive breast cancer. It may rarely cause cardiac side effects. It is very important to make patients aware of these issues and to be consulted by cardiology in case of possible cardiac symptoms.