Transformation of IgM-MGUS into Waldenström´s macroglobulinemia in two of six patients treated for Schnitzler´s syndrome Schnitzler´s syndrome is a very rare, adult-onset, apparently acquired autoinflammatory disease. Chronic urticarial rash and symptoms of systemic inflammation including fever, arthralgia and bone pain with the presence of monoclonal immunoglobulin M (IgM), rarely IgG, are among hallmarks of the disease. We performed a retrospective study of 6 patients (5 men, 1 woman) diagnosed with Schnitzler´s syndrome fulfilling the Strasbourg criteria who had been treated at our centre in the University Hospital Brno from 2007 to 2021. Median age at diagnosis was 54 (45-67) years, median follow up was 8 (3-14) years. All 6 patients had IgM κ monoclonal gammopathy, increased CRP and/or erythrocyte sedimentation rate and arthralgia or bone pain, 4 patients suffered from fever, three had leucocytosis 10 × 109/L and lymphadenopathy was found in one patient. 18FDG-PET/CT scan with low-dose total body CT became a part of the initial baseline assessment in 5 patients with suspected Schnitzler´s syndrome, while Na18F-PET/CT was used in one patient to confirm the presence of osteosclerotic leasions as a criterion of the disease. All patients had osteosclerotic or hyperostotic bone lesions detected by low-dose CT examination, with increased 18FDG uptake in illiac and femoral bone marrow. The patient with Na18F-PET/CT scan revealed intensive abnormal tracer uptake with Na18F-PET/CT being more sensitive for detection of osteosclerotic lesions in Schnitzler´s syndrome than 18FDG-PET/CT. All patients were treated with daily subcutaneous anakinra without any adverse events, with excellent clinical results. We observed complete disappearance of urticaria and other symptoms persisting during years of anakinra administration. IgM-MGUS transformed into Waldenström´s macroglobulinemia in two of six patients, but only one patient developed symptoms requiring RBD (Rituximab, Bendamustin, and Dexamethasone) treatment, which induced almost complete remission of the disease. Successful RBD therapy enabled to prolong intervals of maintenance anakinra from 24 to 48 hours with almost complete control of urticarial rash and other symptoms. We suggest close monitoring of patients with Schnitzler´s syndrome to early capture potential transformation into Waldenström´s macroglobulinemia with succesful treatment of both conditions.