The mitotic indices of the eosinophilic precursors in the bone marrow of 3 untreated patients with eosinophilic leukaemia were found to be significantly lower than those of 11 patients with reactive eosinophilia and those of 15 healthy controls. The same differences were found in the neutrophilic precursors and in the myeloblasts indicating leukaemic involvement of both the eosinophilic and neutrophilic cell lines. Mitotic countings therefore seem to be of diagnostic significance in this rare disease.