BACKGROUND: Hypereosinophilic syndrome (HES) is an infrequent multisystemic disorder with a serious prognosis, defined by persistent marked eosinophilia (>
1500 eosinophils/mm3) associated with organ damage from eosinophil-mediated cytotoxicity. Cardiac involvement is a significant and unpredictable complication of hypereosinophilic syndrome, particularly prevalent in patients carrying the FIP1L1-PDGFRA fusion. CASE SUMMARY: Reported is a case of chronic eosinophilic leukaemia (CEL) with a FIP1L1-PDGFRA rearrangement, diagnosed in a 31-year-old patient presenting with Loeffler endocarditis. Intracardiac thrombi and embolic cerebral infarctions complicated the case. The patient demonstrated haematological remission following chemotherapy, and anticoagulation treatment led to thrombi resolution. DISCUSSION: This case highlights that Loeffler endocarditis can present as the primary and sole manifestation of chronic eosinophilic leukaemia. Effective collaboration between cardiologists and internists is crucial for timely diagnosis and comprehensive management, ultimately resulting in enhanced outcomes.