Sweet's syndrome (acute febrile neutophilic deromatosis) is a rare but not exceptional disease. Its first description was done in 1964, and its diagnosis is now well defined. A few days after a common upper respiratory infection, out patient presented erythematous plaques on the head, neck and forearms, with fever and general malaise. A skin biopsy showed a neutrophilic infiltrate. Oral steroids were prescribed, and healing occured within 48 hours. Respiratory and gastointestinal infections are frequently associated with Sweet's syndrom. Other associations have been described: neoplasias (mainly hemoproliferative diseases), inflammatory diseases, pregnancy, drugs. Cases classified as 'idiopathic' need a close follow-up: Sweet's syndrome may announce a malignant disease. There are also intermediate forms with other neutrophilic dermatoses, e.g. pyoderma gangrenosum.