A 47-year-old white man who worked as a professional musician presented to his general practitioner with a 3-week history of malaise, loss of appetite, slight weight loss, and dark urine. There was no history of respiratory symptoms and, in particular, no hemoptysis. He had had mild asthma for 10 years, which had been treated with inhaled bronchodilators. He was a lifelong nonsmoker and drank alcohol only occasionally. He had no family history of renal disease.The general practitioner detected proteinuria and hematuria, and blood tests showed an elevated serum creatinine. The patient was referred to our renal unit the same day. On arrival, he looked well and was afebrile with no rash or edema. Pulse was 80 beats/minute and regular, and blood pressure was 145/70 mm Hg. Clinical examination of the heart, lungs, abdomen, and nervous system was normal. Urinalysis showed 2ϩ blood, 3ϩ protein, dysmorphic red cells, and granular and red cell casts. Serum creatinine was 550 mol/L (6.3 mg/dL) and urea 21 mmol/L. Electrolytes were in the normal range, but albumin was reduced at 2.3 g/dL. Blood picture showed hemoglobin 10.7 g/dL; white blood cell count 9.6 ϫ 10 9 /L; and platelets 654 ϫ 10 9 /L. Oxygen saturation with the patient breathing room air was 94%, and a chest radiograph was normal.An enzyme-linked immunosorbent assay (ELISA) for antiglomerular basement membrane (anti-GBM) antibodies was