Sweet syndrome is a rare skin disorder characterized by fever, inflammation of the joints (arthritis), and the sudden onset of a rash. The rash consists of bluish-red, tender papules that usually occur on the arms, legs, face or neck, most often on one side of the body (asymmetric). In approximately 80 percent of cases, Sweet syndrome occurs by itself for no known reason (idiopathic). In 10 to 20 percent of cases, the disorder is associated with an underlying malignancy, usually a hematologic malignancy such as certain types of leukemia. The exact cause of Sweet syndrome is unknown.