Objective: Vasculitis is an increasingly prevalent condition, often characterised by an insidious and multifaceted clinical course, which delays accurate diagnosis and treatment. Case report: A 63-year-old male diagnosed with granulomatosis with polyangiitis (GPA), who demonstrated pulmonary disease progression after six months of remission-induction therapy with cyclophosphamide. Imaging studies revealed new cavitating infiltrates in the upper lobe of the left lung, along with numerous satellite nodules in both lungs. A thorough differential diagnosis ruled out infectious aetiologies. Summary: Despite extensive diagnostic efforts, including invasive procedures and positron emission tomography–computed tomography scanning, exclusion of malignancy remained challenging. Establishing an accurate diagnosis and excluding infectious or neoplastic diseases enables timely and appropriate treatment, which can be life-saving in patients with GPA. Delayed treatment increases the risk of GPA-related organ complications.