Abstract Aims Eosinophilic heart disease (EHD) is a rare cardiac condition with a wide spectrum of phenotypes. The diagnostic and prognostic value of cardiac magnetic resonance (CMR) in EHD remains unknown. Methods and results This was a retrospective analysis of 250 patients with eosinophilia referred for a CMR scan (period 2000–2020). CMR data sets and clinical/laboratory data were collected. Patients were followed up for a mean of 24 months (range 1–224) for the composite endpoint of death, acute coronary syndrome, hospitalization for acute heart failure, malignant ventricular arrhythmias, or the need for implantable cardiac defibrillator/pacemaker. The main objectives were to explore the diagnostic value of CMR in EHD; relationships between cardiac function, late gadolinium enhancement (LGE), and EHD phenotypes; and the prognostic value of fibrosis and oedema by CMR. The prevalence of findings compatible with EHD was 39% (patients with cardiac symptoms: 57% vs. screening: 20%, P < 0.001). EHD phenotypes included subendocardial LGE (n = 58), mid-wall/subepicardial LGE (n = 26), pericarditis (n = 5) or dilated cardiomyopathy (n = 8). Myocardial oedema was present in 10% of patients. Intracardiac thrombi (7%) were associated with EHD phenotype (χ2=47.3, P = 1.3×10−8). LGE extent correlated with LVEDVi (rho = 0.268, P = 5.3×10−5) and LVEF (rho=−0.415, P = 8.6×10−11). A CMR scan positive for EHD [hazard ratio (HR) = 5.61, 95% confidence interval (CI): 1.82–17.89, P = 0.0026] or a subendocardial LGE pattern (HR = 5.13, 95% CI: 1.29–20.38, P = 0.020) were independently associated with the composite clinical endpoint. Conclusion The diagnostic yield of CMR screening in patients with persistent eosinophilia, even if asymptomatic, is high. The extent of subendocardial fibrosis correlates with LV remodelling and independently predicts clinical outcomes in patients with eosinophilia.