A 35-year-old woman received an implantable cardioverter-defibrillator (ICD) for primary prevention of sudden cardiac death. One day after implantation, transesophageal echocar­dio­graphy (TEE), carried out before defibrillation threshold testing, revealed a large, mobile, grape-shaped mass attached to the defibrillator lead in the right atrium (RA), fortunately with no symptoms. Anticoagulant therapy was started immediately. Serial TEE demonstrated that, although still asymptomatic, the RA mass was enlarged. Accordingly, thrombectomy, lead extraction and placement of an epicardial ICD lead were per­formed. Correct pacing and defibrillation thresholds were obtained. The patient was discharged 4 days later and during 1 month follow-up remained in good condition with no recur­rence of RA thrombosis.