A previously healthy 17-year-old Caucasian male patient, with no family history of known rheumatic diseases, was observed in the emergency department with a history of fever, dry cough, and pleuritic chest pain for the past 24 h. Chest X-ray revealed an hypotransparency on the left inferior lobe, and antibiotic therapy was initiated, based on the suspicion of community-acquired pneumonia. Five days later, the symptoms worsened, with the onset of conjunctival hyperemia, odynophagia, asthenia, anorexia, and nocturnal diaphoresis. In the emergency department, his weight was 70 kg (had decreased 4 kg compared to two months earlier), and physical examination revealed an axillar temperature of 37.6 C, bilateral bulbar conjunctival hyperemia, and bibasal decreased breath sounds. He was normotensive, and there was no identifiable organomegaly or adenopathy. Laboratory testing revealed hemoglobin 12.3 g/dL (MCV 80 fL, RDW 12.4%), lymphopenia (950/muL), increased DHL (498 U/L), and increased inflammatory markers (reactive C-protein 106.5 mg/L and erythrocyte sedimentation rate 71 mm/h). Chest X-ray exposed multiple nodular hypotransparent lesions scattered by both the lungs, and a CT scan showed parenchymal nodular areas with internal cavitation (Figures 1(a) and 1(b)). Faced with the possibility of a necrotizing pneumonia, he was admitted in the pediatric department and started empirical therapy with ceftriaxone and vancomycin. However, fever persisted, and eight days later, he started complaining of diffuse abdominal pain. A new thoracoabdominal CT scan presented worsening pulmonary lesions and an enlarged spleen (13 cm larger diameter), with marked heterogenicity. Renal function was normal (plasma creatinine 0.8 mg/dL, urea 25 mg/dL), and urinalysis presented a protein count of 25 mg/dL, with 10-25 erythrocytes per high resolution field. He underwent a bronchofibroscopy, which showed a lesion in the left portion of the carina and whose biopsy revealed a necrotizing inflammatory process. An autoimmune workup showed a positive C-ANCA (1/640) and anti-PR3 (2250 UQ) with negative antinuclear antibodies (ANAs), anti-double-stranded DNA (dsDNA), and antimyeloperoxidase (MPO) and a normal immunoglobulin and complement assessment. Considering a systemic ANCA-positive vasculitis, he started prednisolone 60 mg/day. Due to an extensive popliteal deep venous thrombosis and a superficial femoral thrombosis, anticoagulation therapy was also instituted. Five days later, an acute pulmonary thromboembolism and a splenic infarction were diagnosed, leading to intravenous methylprednisolone pulses (1 g for 3 days) and immunoglobulin perfusion (2 cycles of 0.4 g/kg, 5 days). To exclude pulmonary tuberculosis, a cultural and PCR search for BK was carried out. After negative results, rituximab was initiated (375 mg/m2/week, 4 weeks). Pneumocystis jirovecii prophylaxis with cotrimoxazole was also introduced. An episode of macroscopic hematuria was also noticed that resolved after 24 h. Cardiac evaluation revealed a noncircumferential pericardial effusion, without hemodynamic compromise. Ophthalmologic and ear-nose-throat (ENT) evaluations were normal. During outpatient follow-up, an angio-CT scan revealed complete resolution of pulmonary thromboembolism and nearly complete resolution of cavitated granulomas 4 months after onset of the disease (Figures 1(c) and 1(d)), and at this moment, clinical remission was achieved. The anticoagulation treatment was suspended after 9 months, with a progressive tapering of oral glucocorticoid and suspension after 12 months. He underwent maintenance therapy with rituximab every 6 months during the first 2 years. Despite hypogammaglobulinemia (minimum of 496 mg/dL), no infections or complications were reported, and immunoglobulin replacement therapy was not required. During the 43 months of follow-up, there is a reference to mild sinusitis symptoms with no further complaints. Pulmonary function tests and renal function remained normal. Regular analytical monitoring demonstrated normalization of ESR and a progressive decrease of C-ANCA titers.