Two patients with a myeloid malignancy in whom Sweet's syndrome (acute febrile neutrophilic dermatosis) was diagnosed, are described. They suffered from fever and showed cutaneous lesions, with infiltration of the skin by mature neutrophils without signs of vasculitis. In one of them the clonal origin of the infiltrating neutrophils could be demonstrated by in situ hybridization. In this patient an association with the use of recombinant human granulocyte-colony stimulating factor was suspected. In the other patient, Sweet's syndrome was the initial symptom of haematological disease. Inadequate wound healing after surgical procedures led to the diagnosis.