The case history of a 75-year-old woman, who was hospitalized with the diagnosis of an acute erosive colitis, is presented. The patient was treated with hysterectomy for an endometrial cancer in 2000 and had suffered from multiple sclerosis for 15 years. A persistent non-productive cough with fever requested a pneumological consultation. Multiple small alveolar opacities and cavitating lesions were found at chest imaging, but no precise diagnosis was possible. Only 3 weeks after hospitalization, we noticed that a urine analysis had been forgotten. This additional test clearly demonstrated a nephritic sediment and further analysis confirmed the diagnosis of a ANCA-positive microscopic polyangiitis, which promptly responded to immunosuppressive therapy. The necessity of a routine urine analysis in the majority of internal medicine patients and the possible link between small vessel vasculitis and multiple sclerosis are discussed.