Respiratory bronchiolitis-associated interstitial lung disease (RBILD) is a distinct clinicopathologic disease described almost exclusively in cigarette smokers.(1) (2) The disease usually presents with mild symptoms and is associated with a good prognosis. (2) Severe lung dysfunction has not been reported with RBILD, which is often confused clinically and radiographically with desquamative interstitial lung disease or idiopathic pulmonary fibrosis (IPF). Two patients with RBILD who developed severe dyspnea, hypoxemia, and clubbing are described. Initially, IPF was diagnosed in both patients. The severity of symptoms was such that the first patient's room air saturation was 85% and the second patient had severe impairment of lung function, with FEV(1) of 39% and FVC of 40%. Advanced lung disease required supplemental home oxygen therapy in the first patient and referral for lung transplant evaluation in the second patient. After a detailed review of histology revealed a diagnosis of RBILD, both patients were encouraged to stop smoking; smoking cessation led to considerable improvement in symptoms and lung function tests. We conclude that advanced lung dysfunction occurs in some patients with RBILD and should not dissuade that diagnosis.