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We report a case of Sweet's syndrome associated with monoclonal gammopathy of uncertain significance (MGUS) and positivity for perinuclear antineutrophil cytoplasmic antibodies (p-ANCA). A 65-year-old man patient came to our hospital complaining of remittent-intermittent fever, weight loss, associated with papules affecting all his body and without mucosal or ocular involvement. Histology of biopsy specimens of papules allowed the diagnosis of Sweet's syndrome. This is the first case of this rare syndrome associated both with MGUS and p-ANCA positivity. The monoclonal immunoglobulin, possibly directed to neutrophils like an antibody, may cause both their fragmentation and release of antigens responsible of p-ANCA appearance.

A 57-year-old man was admitted due to rapidly progressive renal failure and pulmonary edema. Chest X-ray showed a bilateral lung infiltrate, while a normal myocardial contractility was reported by echocardiography. Though initially ANCA were absent, a necrotizing vasculitis with polymorphonuclear leukocyte infiltrate was observed in a kidney biopsy specimen. Renal histology was compatible with microscopic polyangiitis because of necrotizing lesions located at small vessels and at glomeruli that were not crescentic. Corticosteroids and immunosuppressive treatment was played with significant clinical improvement. Six months later, the patient died of a gastrointestinal hemorrhage due to bowel perforation by vasculitic lesion. This time high p-ANCA positivity was detected and at renal histology crescentic glomeruli were observed.