Clinical, hematological, cytogenetic and pathohistological findings in 14 patients with high eosinophilia allowed the authors to distinguish 2 groups of patients: with symptomatic (secondary) and idiopathic hypereosinophilic syndromes (6 and 8 patients, respectively). The latter was characterized by hepato- and splenomegaly, specific cardiac lesion (thromboplastic endocarditis), non-infectious fever, anemia and thrombocytopenia, marked hypercellularity of the bone marrow with inhibition of erythro- and megakaryocytopoiesis. Ph'-chromosome occurred in 2 out of 8 cases. Biopsy and autopsy histology in all cases of idiopathic hypereosinophilic syndrome were typical for myeloproliferative diseases. In symptomatic hypereosinophilic syndrome the above features were not registered.