[Malignant hypereosinophilic syndrome. Report of an exceptional case (author's transl)].

The case of a 43-year-old man with a highly malignant hypereosinophilic syndrome is reported. The condition is classified as such according to Hardy and Anderson's criteria, accepted by many other authors. Other diseases of known etiology which may present high levels of eosinophils in the peripheral blood, such as parasitosis, allergies, neoplasias, collagenosis, etc., were discounted beforehand. The difficulties in distinguishing between these diseases are discussed; they are often accompanied by clinical manifestations which also arise in very different conditions including eosinophilic leukemia, Engfeldt and Zetterström's eosinophilic collagenosis, Löffler's fibroplastic endocarditis, etc. A particularly striking feature of this condition is the formation of large tumor masses of mature eosinophils. They begin in various bones, which they destroy almost completely, and invade the surrounding tissues, destroying them as well. These tumors act similarly to malignant eosinophilic myelocytomas, a fact which has not been reported previously in the literature as far as we know. Although the eosinophils act as though they were neoplastic, they maintain the characteristics of mature cells, both cytomorphologically and ultrastructurally as well as cytochemically (consistently chloroacetate esterase negative). The tendency to diagnose eosinophilic leukemia solely on the basis of the malignancy of the condition and a tissue infiltration of eosinophils without determining the existence of cytologic and/or cytochemical anomalies of the cells showing them to be leukemic is discussed. The authors were unable to find any reports in the literature in which the eosinophils were presented with unmistakeably blastic cellular characteristics. Various nosologic considerations are offered.