Sweet's syndrome (SS) is characterized by the clinical features of fever, leucocytosis, neutrophilia and the sudden onset of asymmetric, often very painful skin lesions and dense dermal infiltrates of mature neutrophils without signs of vasculitis. Apart from idiopathic cases the disease is frequently associated with hematological malignancies, but it may also be observed in patients with solid tumors, mainly tumors of the genito-urinary tract. In the past, numerous theories have been proposed to explain the pathogenesis of this rare disease. SS has been interpreted as a direct response to mechanical and chemical irritants, an infectious disease or a disorder of neutrophilic chemotaxis and/or phagocytosis, but most often it has been described as a hypersensitivity reaction. Each of these theories can account for particular symptoms, but none of them reconciles the dominating clinical and laboratory features of the disease. Furthermore recently published casuistic observations suggest the involvement of certain cytokines in particular G-CSF and Il-6 in the pathogenesis of the disease, which might explain many of the observed clinical and laboratory findings. The following article summarizes these data and gives a review of the current literature.