A 58-year-old man was diagnosed with stage IV (cT2N0M1) non-small-cell lung cancer comprising adenocarcinoma. The patient had no past medical history, including cardiovascular diseases. Prior to anticancer therapy, jugular venous distension and limb oedema were not detected, and heart sounds were normal. Electrocardiography demonstrated normal sinus rhythm, and computed tomography (CT) revealed no cardiomegaly or pericardial effusion. Lung cancer was aggravated despite anticancer chemotherapies, including cisplatin and pemetrexed as the 1st-line, docetaxel as the 2nd-line, tegafur/gimeracil/oteracil as the 3rd-line, and amrubicin as the 4th-line therapy. Nivolumab (3 mg/kg every 2 weeks), a programmed death-1 (PD-1) inhibitor, was administered as 5th-line therapy. Eighteen months after the initiation of nivolumab (after the 35th-cycle), the patient developed fatigue, limb oedema, and increased body weight (BW). Physical examination revealed jugular venous distension and limb pitting oedema. The serum creatinine level was 0.77 mg/dL (normal range: 0.6–1.0 mg/dL). No electrocardiographic abnormalities were found. On echocardiography, ventricular systolic function on both sides and left ventricular diastolic function were preserved (abnormal relaxation pattern, ), but mild pericardial effusion was detected (). The diameter of the pericardial echo-free space was 9 mm in the anterior right ventricle. CT revealed pericardial thickening, and trivial effusion in the pericardium and pleura (). As the patient was haemodynamically stable and the volume of pericardial effusion was mild, pericardiocentesis was planned to be performed if effusion increased. Favourable response of the lung cancer to treatment on CT suggested a non-malignant cause. Viral or purulent pericarditis was unlikely because the patient had no fever, a normal white blood cell count (6570/μL; normal range: 3300–8600/μL), and a negative blood culture test. Autoantibodies were all negative. Liver biopsy for the examination of liver enzymes revealed T-cell lymphocytic infiltration. Therefore, the patient was diagnosed with irAE hepatitis. Based on these findings, the patient was diagnosed with irAE pericarditis with constrictive features. Nivolumab was suspended and a high-dose corticosteroid, prednisolone at 0.5 mg/kg, was initiated. Right heart failure (RHF) symptoms were gradually ameliorated and the serum C-reactive protein level decreased from 4.08 mg/dL to 1.93 mg/dL (normal range: 0.00–0.14 mg/dL). However, RHF was exacerbated during the tapering of prednisolone (, 1st RHF exacerbation). Therefore, the dose of prednisolone was increased to the initial dose and re-tapered carefully. At the time of cessation of prednisolone, pericardial effusion was significantly reduced (). For a few months, the BW gradually increased with furosemide at 10 mg/day and quickly increased 3 months after the cessation of prednisolone (, 2nd RHF exacerbation), and the patient was positive for Kussmaul’s sign. Trivial pericardial effusion and massive pleural effusion were noted on CT (). In addition, echocardiography revealed pericardial adhesion (), aggravation of left ventricular diastolic function to a restrictive pattern (), significant reduction of the mitral peak E wave velocity in the inspiratory phase (), and septal bounce (Video 1). A higher peak e′ velocity on tissue Doppler (>8 cm/s) suggested a lower likelihood of restrictive cardiomyopathy (). Right heart catheterization demonstrated a W-shaped right atrial pressure waveform (). Simultaneous recording of biventricular pressures revealed a dip and plateau pattern in both ventricles, low end-diastolic pressure difference between the left and right ventricle (1 mmHg; cut-off: 5 mmHg), and high systolic area index (1.11; cut-off: 1.1), which suggested discordant changes in right and left ventricular filling during respiration (). As the amount of pericardial effusion was slight, pericardiocentesis was not performed. The serum troponin-T level was high (0.123 ng/mL; normal range: <0.014 ng/mL), but there was no evidence of active myocarditis by endomyocardial biopsy. Cardiac magnetic resonance imaging demonstrated pericardial thickening, but there was no myocardial oedema or significant late gadolinium enhancement in the pericardium or myocardium. At this point, the patient was diagnosed with constrictive pericarditis (CP) with myopericarditis. The dose of furosemide was increased to 40 mg/day and tolvaptan at 15 mg/day was added, but these did not ameliorate RHF symptoms. Consequently, methylprednisolone at 1 g/day for 3 days and subsequent prednisolone at 1 mg/kg were administered. RHF symptoms were transiently ameliorated, but they exacerbated again after the tapering of prednisolone (, 3rd RHF exacerbation). As pericarditis recurred repeatedly and progressed to CP despite corticosteroid therapy, infliximab (5 mg/kg) was initiated. Infliximab was administered 2 and 6 weeks after the 1st administration, and every 8 weeks thereafter in accordance with the regimen for the treatment of connective tissue disease (). After the initiation of infliximab, the BW decreased from 82 to 75 kg despite a reduction in the dose of diuretics (tolvaptan 7.5 mg/day), and the serum troponin-T level decreased to 0.020 ng/mL. Although CP findings were still present by echocardiography (), prednisolone was able to be tapered to a lower dose (0.15 mg/kg) without aggravation of RHF; therefore, pericardiectomy was not performed. The patient was administered low-dose prednisolone and repeated infliximab for 11 months after the 3rd RHF exacerbation without worsening RHF ().