Granulomatosis with polyangiitis (GPA) is a rare disease with inflammation in the small vessels. This is a case report of GPA, where lung cancer was initially suspected. A 54-year-old woman eventually developed manifestations from several organ systems, and a diagnosis of GPA was confirmed. Treatment was initiated, but the neurological manifestations proved to be irreversible. This case illustrates that GPA is important to acknowledge, because timely treatment is crucial to reduce mortality and morbidity.