Background   Basophilic crisis and eosinophilia are well recognized features of advanced chronic myeloid leukaemia. In other myeloid neoplasms, however, transformation with marked basophilia and eosinophilia is considered unusual.

Design   We examined the long‐term follow‐up of 322 patients with de novo myelodysplastic syndromes (MDS) to define the frequency of basophilic, eosinophilic and mixed lineage (basophilic and eosinophilic) transformation.

Results   Of all patients, only one developed mixed lineage crisis (≥ 20% basophils and ≥ 20% eosinophils). In this patient, who initially suffered from chronic myelomonocytic leukaemia, basophils increased to 48% and eosinophils up to 31% at the time of progression. Mixed lineage crisis was not accompanied by an increase in blast cells or organomegaly. The presence of BCR/ABL and other relevant fusion gene products (FIP1L1/PDGFRA, AML1/ETO, PML/RARα, CBFβ/MYH11) were excluded by PCR. Myelomastocytic transformation/myelomastocytic leukaemia and primary mast cell disease were excluded by histology, KIT mutation analysis, electron microscopy and immunophenotyping. Basophils were thus found to be CD123+, CD203c+, BB1+, KIT‐ cells, and to express a functional IgE‐receptor. Among the other patients with MDS examined, 4(1·2%) were found to have marked basophilia (≥ 20%) and 7(2·1%) were found to have massive eosinophilia ( ≥ 20%), whereas mixed‐lineage crisis was detected in none of them.

Conclusions   Mixed basophil/eosinophil crisis may develop in patients with MDS but is an extremely rare event.