A patient with common acute lymphoblastic leukaemia (ALL), hypereosinophilic syndrome and t(5;14) (q31.1;q32.3) translocation is described. Even with intensive treatment only short periods of complete remission were achieved. Recurrence of the leukaemia was always accompanied by the appearance of eosinophilic granulocytes in the blood and in the bone marrow. Although there is no experimental proof we assume that the hypereosinophilic syndrome is causally related to the chromosome aberration. Translocation of the GM-CSF gene from chromosome No. 5 to chromosome No. 14, might have led to the deregulation of the gene by enhancer sequences of the immunoglobulin heavy-chain region on chromosome No. 14, with the consequence of an overproduction of neutrophilic and particularly eosinophilic granulocytes. Furthermore, stimulation of the leukaemic cell clone may have occurred by this translocation. The similarity of the clinical course with cases described in the literature suggests that this condition is a unique entity of ALL.