We present a woman with paroxysmal atrial fibrillation and a mass in the left atrium. Cross-sectional echocardiography defined the lesion as cystic, while magnetic resonance imaging depicted it as a solid mass with areas of focal bleeding. Surgical resection showed a partially organized haematoma within the left atrial wall. The patient did well after surgery, although atrial fibrillation recurred. The presence of an atrial haematoma is rare, specially when spontaneous, and we stress the usefulness of magnetic resonance in reaching an aetiologic diagnosis.