Background: Primary cardiac cancer is a rare event with various clinical presentations and often causes unexpected symptoms or sudden death. Case reports with this diagnosis are scarce.

Case presentation: We present an unusual manifestation of leiomyosarcoma of the left atrium in a female patient, 33 years old. Presenting difficulty to walk, dyspnoea at rest, skin pallor, cough with hemoptoics and syncope. A transthoracic echocardiogram showed cavitary enlargement of the left atrium, moderate to significant mitral stenosis with an adherent mass in the anterior leaflet, left ventricular systolic function preserved at rest, and mild aortic and tricuspid insufficiency. The procedure was complete resection of the tumour or negative microscopic margins (R0 resection), 25 sessions of radiotherapy, 5 cycles of adjuvant chemotherapy using gemcitabine (900 mg/m2 on days 1 and 8) and docetaxel (75 mg/m2 on day 8), with a resolution of the clinical picture. After 5 years of follow-up, the patient had no metastases or recurrence of the initial tumour.

Conclusion: The nonspecific symptoms presented in the reported case demonstrate that the cardiac tumour can mimic other cardiac disorders, such as coronary artery disease or pericarditis, rarely representing the first manifestation of a previously unknown malignancy.