Patient 2. A 2-year-old girl developed high spiking fever, rash (flat, non-itchy salmon-pink macules and papules coinciding with fever), arthritis, leukocytosis with neutrophilia, thrombocytosis, increased CRP, ESR, anemia, hyperferritinemia, and increased LDH with a positive effect of systemic corticosteroids. Two months later when the corticosteroids were tapered, she developed a mass lesion of the right anterior chest wall with axillar lymphadenopathy. A repeated chest CT scan revealed a destruction of the 5th right rib with periosteal reaction and an extraosseous mass lesion 25 x 48 x 56 mm under the musculus pectoralis major with foci of contrast uptake (Figure 3). Two surgeries were performed. Specific infections (tuberculosis and mycosis) and malignancy were ruled out. A morphological examination revealed chronic bone inflammation (infiltration with lymphocytes, plasma cells with a small count of neutrophils, and bone sclerosis). A CT revealed rib osteosclerosis. The fever and rash persisted, indicating the poor efficacy of antibiotics. Episodical corticosteroid injections for rash reduced the fever and rash. After 5 months the girl was admitted to our department of immunology and allergology, where she again developed fever and rash, leukocytosis, neutrophilia, and thrombocytosis-increased CRP. Due to possible immunodeficiency, antibiotics were started again, but her condition worsened. She developed a highly persistent non-spiking fever and rash and also a full-blown macrophage activation syndrome:WBC 4.6 x 109/L; platelets 135 x 109/L; Hb 66 g/L, CRP 59 mg/L, AST 105 U/L, LDH 1,263 U/L, ferritin 260 ng/ml, D-dimer 2,486 ng/ml (n.v. < 500), fibrinogen 1.4 g/L). The known immune deficiency was ruled out, and WES did not reveal any relevant findings. Antibiotics were discontinued, and whole-body MRI, chest, and abdominal CT revealed axillar, abdominal, mesenteric, inguinal lymphadenopathy, hepatosplenomegaly, joint effusion, and 5th rib osteosclerosis. There were no lung infiltrates. Due to the inefficacy of antibiotics and the positive effect of corticosteroids administered earlier, anakinra with a dosage of 5 mg/kg/day was initiated, which had good efficacy. In a span of 2 weeks, the patient developed a flare, and in the following 2 weeks, anakinra was switched to canakinumab at a dosage of 4 mg/kg every 4 weeks, which resulted in a complete resolution of the disease. Now she is in remission upon canakinumab treatment.