Morphological features of eosinophils in patients with reactive eosinophilia (28 patients) and clonal eosinophilia (26 patients) have been compared with each other and with the eosinophil characteristics of healthy volunteers (three subjects) and of patients with the idiopathic hypereosinophilic syndrome (three patients). Morphological features, assessed in isolation from other haematological abnormalities, were found to have poor specificity for a myeloid neoplasm. The most useful feature was the presence of basophilic granules in mature eosinophils, which was associated particularly with acute myeloid leukaemia with inv(16). Marked reduction in granules occurred more often in some subsets of the myeloid neoplasm group but nevertheless was lacking in specificity since it was not infrequently seen in reactive eosinophilia. Although experienced morphologists more often considered that a myeloid neoplasm was likely in patients in whom this was the diagnosis (69%), myeloid neoplasia was also considered likely in a considerable proportion (39%) of patients with reactive eosinophilia. Morphological abnormalities of eosinophils therefore cannot be assessed in isolation in seeking to make a diagnosis of a myeloid neoplasm. Morphology is, however, needed and should be integrated with the results of other investigations.