Painful left bundle branch block (LBBB) is characterized by chest pain in setting of transient LBBB, in the absence of ischemia. Management has focused on pharmacological suppression of the sinus rate to prevent LBBB, or invasive therapy with permanent pacing. To determine the feasibility of using oral theophylline for treatment of painful LBBB. N/A A 71-year-old man presented for evaluation of exertional left-sided chest pain that resolved with rest. 12-lead electrocardiogram (ECG) showed sinus bradycardia at 54 bpm. Transthoracic echocardiogram was normal and coronary angiography showed non-obstructive coronary artery disease. Symptoms persisted despite treatment for microvascular angina with beta-blockers and ranolazine. Heart rate and ECG monitoring using an Apple watch demonstrated reproducible symptoms when heart rate increased above 80 bpm and resolution with decrease in heart rate below this threshold. Electrophysiology study demonstrated rate-related left bundle branch block and concurrent onset of symptoms with atrial pacing at a cycle length of 550 ms (∼109 bpm) (Panel A) and prompt resolution of symptoms with narrowing of QRS at lower pacing rates. Transient right bundle branch block was intentionally induced with catheter manipulation (Panel B) and atrial pacing produced 1:1 conduction down the left bundle with right bundle branch block up to 113 bpm with no symptoms and then to 2:1 block above the His (Panel C). Isoproterenol infusion allowed for atrial pacing up to 127 bpm without evidence of LBBB (Panel D). The patient was then discharged on theophylline 100 mg bid, which has a similar chronotropic and dromotropic effect as isoproterenol. He reported significant improvement in functional status with no recurrence in symptomatic LBBB. We report the first case of successful treatment of painful LBBB with oral theophylline. The mechanism may be improvement in conduction in damaged fibers to the left bundle. Theophylline should be considered in patients with painful LBBB in whom sinus node suppression has been unsuccessful and invasive management with permanent pacemaker is not desired.