The HES is characterized by persistent eosinophilia and significant organ damage and is likely of diverse etiology. The relationship of HES to leukemic disorders has been particularly difficult to clarify. We report a case of HES presenting with an eosinophil count in excess of 100,000/mm3, congestive heart failure and bone marrow - but not peripheral blood - lymphoblasts. Parasitic and isohemagglutinin titers were not elevated; IgE was normal. Bone marrow blasts were separated from eosinophils by density gradient and tested for cytologic, immunologic and chromosomal markers. Lymphoblasts possessing neither T nor B cell markers were identified. Quinacrine band fluorescence preparations disclosed that 86% of the metaphases from the bone marrow lymphoblasts contained an extra C group chromosome and a marker chromosome 14 (14 q+). Stimulated and unstimulated peripheral blood chromosomes were normal. Eosinophils and lymphoblasts disappeared under Vincristine, prednisone and 1-asp induction therapy. Chromosome 14 anomalies have been associated with other lymphoproliferative disorders, such as Burkitt's lymphoma, ataxia-telangiectasia and lymphosarcoma, but not myelo-proliferative states. Our results suggest that a “null cell” ALL-llke lymphoproliferative disorder with chromosome 14 marker can induce the HES.