Interstitial Lung Diseases of Unknown Cause: Disorders Characterized by Chronic Inflammation of the Lower Respiratory Tract

THE interstitial lung diseases are a heterogeneous group of disorders of the lower respiratory tract characterized by derangements of the alveolar walls and loss of functional alveolar capillary units.^{1 2 3} More than 100 agents are known to cause interstitial disease, but in two thirds of all cases, no cause can be identified.³ This group, commonly referred to as interstitial lung diseases of unknown cause, is responsible for more than 10,000 hospital admissions in the United States each year and has a prevalence of 5 to 10 cases per 100,000 population.³ The list of interstitial lung diseases of unknown cause includes 35.