Sir: Acute febrile neutrophilic dermatosis or Sweet's syndrome' is an uncommon condition characterised by fever, polymorphonuclear leucocytosis, painful erythematous cutaneous plaques, and a dense dermal infiltrate of neutrophils without vasculitis at the site of the skin lesions. Although many investigators suggest that acute febrile neutrophilic dermatosis is a hypersensitivity reaction,2 no defmitive cause is known. The role of the neutrophil as a cause or effect in this syndrome has not been clarified. Acute febrile neutrophilic dermatosis is histologically and clinically mimicked by several disorders, and the differential diagnosis with pyoderma gangrenosum3 and with some reactive erythemas such as erythema multiforme2 is usually difficult.