The histological features of biopsies from 18 previously unreported cases of Sweet's syndrome are reported. The dermal infiltrate in the majority of the cases contained numerous histiocytes that at first sight appeared to mimic neutrophils. The immunophenotype of these histiocytes was consistent with monocytes that have freshly infiltrated into the lesions. Only two of the cases in this series, associated with leukaemia, displayed the histological features of Sweet's syndrome with a predominant neutrophilic infiltration. We suggest that the initiating mechanisms in Sweet's syndrome are that monocyte/histiocyte-derived cytokines such as the interleukins IL-1 and IL-8, secreted either by infiltrating histiocytes in the non-leukaemia-associated cases of Sweet's syndrome or by tumoural myelomonocytic cells in those associated with leukaemia, are responsible for the systemic manifestations and the infiltration with neutrophils in the skin lesions.