Tachy-brady syndrome or sick-sinus syndrome as it is also known is a cardiac rhythm disturbance resulting in alternating episodes of bradycardia and tachycardia. Diagnosis can be difficult because of its nonspecific symptoms and elusive findings on electrocardiogram or 24h tape. Thymic cysts are relatively uncommon tumours that are predominantly asymptomatic and located in the anterior mediastinum. We present the first known report of tachy-brady syndrome associated with a large thymic cyst. Treatment consisted of dual-chamber pacemaker implantation prior to video-assisted removal of the thymic cyst.