BACKGROUND Immuno-oncology (IO) therapy is often associated with immune-related adverse effects (irAEs) varying in severity and involving different organs. There are many common irAEs; however, the rare irAEs include myocarditis, pericarditis, colitis, pneumonitis, dermatitis, and hepatotoxicity, which are associated with higher rates of morbidity and mortality. Approximately 0.06%–2.4% of SCLC patients develop IO-related myocarditis. CASE PRESENTATION A 74-year-old female with a past medical history of hypertension, O2-dependent COPD, pAF, SCLC, and a 40-pack-year history of tobacco abuse presented to the hospital with chest and back pain. Her pain radiated towards the epigastric region, and she was also experiencing nausea. She received two cycles of carboplatin/etoposide followed by two cycles of chemoradiation, during which substantial tolerance issues were noted, even with dose reductions and treatment delays due to AF, respiratory failure, and influenza. After deferring therapy, her condition relapsed within several months, and she was started on Nivolumab monotherapy four weeks prior to her admission. Upon presentation, it was noted that the patient’s troponin T was elevated (177 ng/L). She also had an abnormal NT-proBNP (10,279 pg/ml), and the ECG suggested NSTEMI changes. The CT thoracic spine reports indicated a T9 compression fracture, which correlated with the site of back pain. For the concurrent chest pain, a TTE was performed, showing depressed RV systolic function, grade I diastolic dysfunction, and LVEF of 45-49% (previous LVEFs of 50–54%). Since multiple abnormalities were noted on TTE and ESR, CRP were elevated, and a cardiac MRI was performed. Due to a drop in EF, a diagnosis of immune-mediated myocarditis due to Nivolumab was suspected. The patient was started on Solumedrol-1 mg/kg, resulting in clinical improvement. A cardiac MRI was performed four days after initiation of treatment, demonstrating wall motion abnormalities in the mid/distal septal wall with patchy segments of non-CAD LGE in the mid-distal septum and apical septal wall. Findings were consistent with IO-associated myocarditis. DISCUSSION PD-1 inhibitors, such as Nivolumab, represent an effective treatment option for SCLC that induces irAEs. Myocarditis associated with ICIs necessitates immediate and continuous monitoring after starting immunotherapy. This case seeks to raise awareness for early detection and intervention, which are essential in managing ICI-induced myocarditis. Elevated cTnI levels and ECG changes are key biomarkers. Frequent ECG changes include T waves, ST segment abnormalities, and arrhythmias. As seen in this case, high doses of glucocorticoids are effective in alleviating symptoms, especially in cases of severe myocarditis.