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The interpretation of pulmonary function tests (PFTs) in patients with connective tissue disease poses considerable difficulties in clinical practice. In established lung disease, several characteristic PFT profiles are recognized for individual complications, including pulmonary fibrosis, pulmonary vascular disease, extrapulmonary restriction, and intrinsic airway disease. PFTs play a central role in the investigation of respiratory symptoms and are sensitive in the detection of interstitial lung disease and pulmonary vascular disease. The staging of disease severity using PFTs is an invaluable guide to prognosis. Maximal exercise testing has an ancillary role in selected patients and is most often useful in demonstrating that limited disease is not clinically significant. The interpretation of PFTs is most difficult when, as is commonly the case, two or more pulmonary complications coexist, each having separate functional effects. Finally, the use of PFTs in routine monitoring is discussed.