A 20-year-old man receiving chemotherapy for diffuse large B‑cell lymphoma with vascular involvement presented to the emergency room with dyspnoea. Chest radiography showed left-sided pleural effusion and an enlarged cardiac silhouette. Transthoracic echocardiography demonstrated large circumferential pericardial effusion with a so-called swinging heart with inflow obstruction (Fig. 1a). Remarkably, the patient was bradycardic and follow-up electrocardiography (ECG) revealed a junctional escape rhythm. Symptoms resolved after pericardiocentesis and drainage of 1,100 ml pericardial fluid containing B cells. Despite the drainage, a junctional rhythm persisted and after 24 h of continuous ECG the patient was discharged. Several weeks later, drainage was repeated for recurring pericardial effusion. ECG then showed an atrial flutter with 2:1 conduction. While in remission, follow-up electrocardiograms showed various ectopic atrial foci rhythms before returning to sinus rhythm 6 months later (Fig. 1b). Cardiac involvement of lymphomas is not uncommon. However, when patients develop transient blocks or arrhythmias this can be life-threatening and require additional vigilance during management [1‐3].