“…HES is a subset of idiopathic eosinophilia that fulfils the traditional criteria of a persistent (>6 months) increase in AEC (>1·5 × 10 9 /l) associated with target organ damage (Chusid et al , 1975). However, evidence from both X‐linked clonality studies (Chang et al , 1999; Malcovati et al , 2004) and long‐term follow up of affected patients suggest that at least some patients with HES harbour an underlying clonal myeloid malignancy that could progress into frank acute leukaemia or MPD (Owen & Scott, 1979; Yoo et al , 1984; Brown & Stein, 1989; Needleman et al , 1990). On the other hand, the demonstration of a clonal (Simon et al , 1999) or phenotypically abnormal (Simon et al , 1999) T‐cell population in other patients with HES suggests the alternative possibility of a true secondary process with some of these patients progressing into clinically overt lymphoma (Butterfield, 2001).…”