BACKGROUND: Immune checkpoint inhibitors are new immunotherapy drugs globally used for many malignancies, including renal cell carcinoma. Myocarditis as an immune-related adverse event is rare but highly fatal, suggesting that its frequency may be higher than reported. This paper describes a case of myocarditis that developed asymptomatically following ipilimumab and nivolumab combination therapy for renal cell carcinoma. CASE PRESENTATION: A 71-year-old Asian man who presented to hospital with fever, fatigue, and weight loss of approximately 10 kg within 2 months was diagnosed with Xp.11.2 translocation renal cell carcinoma. Computed tomography revealed multiple lung masses, mediastinal lymph node enlargement, and a level II tumor thrombus reaching the inferior vena cava (cT3bN0M1; International Metastatic Renal Cell Carcinoma Database Consortium, poor risk). Ipilimumab/nivolumab combination therapy was started as induction therapy. The patient experienced acute interstitial nephritis as an immune-related adverse event after treatment initiation; however, a good response to steroid therapy was observed. The antitumor effect of the immunotherapy was notable. Although he experienced pulmonary embolism, it seemed asymptomatic and harmless; thus, a second infusion was introduced. From the eighth day, he demonstrated rapidly worsening cardiogenic shock with asymptomatic electrocardiographic changes and drastic drop in cardiac biomarkers, and a diagnosis of myocarditis as an immune-related adverse event was made. Although immediate methylprednisolone mini-pulse therapy followed by tapered prednisolone prevented mortality, extensive myocardial fibrosis with marked ejection fraction decline persisted as a sequela. Consequently, follow-up without treatment was instituted; however, much of the tumor response initially observed was maintained over several months. CONCLUSION: Physicians treating patients with immune checkpoint inhibitors should be aware of their potentially life-threatening cardiotoxic effects. This study emphasized the importance of a high index of suspicion, prompt diagnosis, and early intervention in patients who present with cardiac abnormalities and possible myocarditis after receiving immunotherapy.