A 15-year-old boy presented to our department with a 5-d history of fever (37.8-38.6 °C) and chest pain. The boy had a 2-year history of histologically-proven extensive UC, for which mesalamine was effective to achieve remission without adverse reactions. Two weeks prior to presentation, he experienced 7-8 bloody stools per day after eating ice cream, at which time he had been taking mesalamine (3 g/d) as maintenance therapy for a year. Given a likely gastrointestinal infection, cefixime (0.2 g/d) was added. His diarrhea and bloody stools improved quickly from 7-8 times a day to twice a day, which confirmed our diagnosis of infection. However, without any antibiotic changes, he developed modest fever, progressive pleuritic chest pain, and shortness of breath after activity. The patient had not received any coronavirus disease 2019 vaccinations. There was no personal nor familial history of cardiac abnormality or dysfunction. On admission, the patient’s vital signs were stable and general examination was unremarkable. A 12-lead electrocardiogram (ECG) demonstrated sinus tachycardia of 102 bpm without other abnormalities. Laboratory tests revealed raised cardiac biomarkers [cardiac troponin I (cTnI) 1.27 ng/mL, N-terminal (NT)-pro hormone brain natriuretic peptide (BNP) 303 pg/mL], and acute phase reactants [high-sensitivity C-reactive protein (hsCRP) 64.7 mg/L and erythrocyte sedimentation rate (ESR) 67 mm/h]. Other myocardial enzymes, including lactate dehydrogenase 121 U/L and creatine kinase MB (CKMB) isoenzyme 0.8 μg/L, were normal. Complete cell counts were roughly normal, except for slight leukocytosis and anemia (white blood cells 10.9 × 109/L, neutrophils 9.1 × 109/L, and hemoglobin 113 g/L). Trans-thoracic echocardiogram (echo) revealed only trace pericardial effusion and a left ventricular ejection fraction of 66%. No hypokinesia or ventricular dilation was seen. Given the clinical presentation in the absence of cardiovascular risk factors, a diagnosis of acute myocarditis was suspected, and the etiology was initially considered to be an infection. He was treated with trimetazidine and potassium and magnesium supplementation. His clinical condition gradually improved, with cTnI decreasing to 0.84 ng/mL over the next 5 d. Cardiac magnetic resonance (CMR) imaging was performed. T1 mapping showed diffuse, slightly elevated T1 values consistent with probable myocarditis[]. To identify the cause of myocyte injury, stool examination, blood cultures, and extensive viral serology for cytomegalovirus, coxsackie virus, and Epstein-Barr virus were checked and were negative. Endoscopic evaluation of the colon with biopsies revealed active UC (Mayo score 3) involving the colon from the hepatic flexure distally, although the patient did not describe any worsening of gastrointestinal symptoms.