Hypereosinophilic syndromes may result either from eosinophilic differentiation of a clone of neoplastic cells or from reactive eosinophilia. In other patients HES is idiopathic. It appears likely that in many patients the "idiopathic' hypereosinophilic syndrome is actually a chronic myeloproliferative disorder. Those cases showing an increase of blast cells or a demonstrable clonal cytogenetic abnormality should be classified as eosinophilic leukaemia. In other cases the neoplastic nature of the disease can be recognized only in retrospect when a granulocytic sarcoma or AMI, develops. A few cases of idiopathic HES are consequent on cytokine secretion whereas others remain idiopathic at the time of death. When eosinophilia occurs as a feature of an acute or chronic myeloid leukaemia or a chronic myeloproliferative disorder the eosinophils are usually part of the leukaemic clone. However, eosinophilia in association with acute lymphoblastic leukaemia is usually reactive. Rare cases have a biphenotypic leukaemia/lymphoma with both eosinophils and lymphoid cells arising from a mutant pluripotent stem cell.