Loeffler's endocarditis in a 47-year-old man was followed 5 years. He died of heart failure. His original diagnosis was eosinophilic leukaemia but cytogenetic investigation failed to show any chromosomal aberration and only mature eosinophilic leukocytes were present in peripheral blood. Relation between heart lesions and eosinophilic leukocyte factors (peroxidase, major basic protein, cationic protein, transforming growth factor beta) were discussed.