Lithium induced cardiotoxicity is associated with several electrocardiographic (ECG) findings. The most commonly observed cardiac effects include QT prolongation, Twave abnormalities, and to lesser extent SA node dysfunction and ventricular arrythmias. We present a case of a 13-year-old female with acute lithium overdose whodeveloped Mobitz I, a manifestation of lithium associated cardiotoxity not previously reported. The patient had no significant past medical history and presented to the emergency department 1 h after intentional overdose of 10 tablets of unknown drug. Parents reported that the patient had visited her grandmother, who "regularly took many different kinds of medications," earlier that same evening. On physical examination the patient had reassuring vital signs, was in no acute distress,cardiopulmonary examination was normal, had clear sensorium, and no evidence of a toxidrome. On serological examination complete blood count, chemistries panel, and liver function tests did not show significant derangements. 4 h post-ingestion acetaminophen concentration was 28 mcg/ml and below indication for n-acetylcysteine antidote therapy. During her ED course she showed evidence of Mobitz I (Wenckebach) on 12-lead ECG. No prior ECGs were available for comparison. Medical toxicology was consulted at that time given concern for potential cardiotoxicity from an unknown xenobiotic. Serum dioxin and lithium concentrations were subsequently requested. Serum digoxin concentration was undetectable. Serum lithium concentrations was 1.7 mEq/L (0.6-1.2 mEq/L therapeutic range). The patient was treated with intravenous hydration at twice maintenance rate. Repeat lithium concertation 14 h post-ingestion was undetectable. During her admission, the patient remained hemodynamically stable and asymptomatic despite occasional episodes of Mobitz I, lasting seconds to minutes. Repeat 12-lead ECG obtained 20 h post-ingestion showed normal sinus rhythm. Cardiology recommendations included ambulatory Holter monitoring upon discharge and follow-up in clinic within two weeks. The patient was medically cleared after 36 h of monitoring and discharged after psychiatric evaluation. Our case demonstrates that patients who develop a new Mobitz I atrioventricular block of unclear etiology in the setting of acute ingestion should be screened for lithium exposure, even if otherwise free of more typical manifestations of lithium toxicity.