Respiratory bronchiolitis-associated interstitial lung disease is a distinct clinical syndrome found in current or former cigarette smokers. The disease often is confused with other interstitial lung diseases, especially idiopathic pulmonary fibrosis. The clinical presentation resembles those of patients with other interstitial lung diseases--cough and dyspnea, with coarse rales on physical examination. Diffuse fine reticulonodular interstitial opacities are found on chest radiograph, usually with normal-appearing lung volumes. Bronchial wall thickening, prominence of peribronchovascular interstitium, small regular and irregular opacities, and small peripheral ring shadows are distinctive features. Pulmonary function testing may be normal but usually demonstrates mild to moderate restriction and normal or slightly reduced diffusing capacity. A mixed obstructive-restrictive pattern is common. Respiratory bronchiolitis-associated interstitial lung disease is characterized histologically by an inflammatory process involving the membranous and respiratory bronchioles. The pathologic findings are dominated by the finding of tan-brown pigmented macrophages within respiratory bronchioles and neighboring alveolar ducts and alveoli. The pulmonary parenchyma away from the airway usually is normal or may demonstrate mild hyperinflation. The clinical course and prognosis of respiratory bronchiolitis-associated interstitial lung disease are unknown. Most patients respond favorably to corticosteroids, with documented improvement in lung function and chest radiographs. Smoking appears to play a role in the pathogenesis, so smoking cessation is important in the resolution of this syndrome.