BACKGROUND: The lung function of patients with idiopathic pulmonary fibrosis (IPF) has not been characterized in detail.OBJECTIVE: To characterize the heterogeneous physiological abnormalities that exist in patients with IPF during their initial clinical evaluation.METHODS: Lung function tests from 93 patients, performed within six months of the initial diagnosis of IPF, were obtained from a referral pulmonary function laboratory at a tertiary care hospital in Canada. A restrictive pattern was defined as total lung capacity (TLC) <95th percentile of predicted value. Patients with obstructive lung disease, lung cancer, emphysema and other restrictive lung diseases were excluded.RESULTS: On diagnosis, 73% of patients with IPF had a restrictive pattern, with a mean TLC of 72% of predicted. Mean forced vital capacity (FVC) was 71% and 44% of patients had an FVC <95th percentile. Mean diffusing capacity for carbon monoxide (DLco) was 60% and DLco/alveolar volume (VA) 92% of predicted. Increased severity of restriction – based on TLC – was associated with lower DLco (74% of predicted in mild restriction and 39% of predicted in severe restriction) and higher forced expiratory volume in 1 s (FEV1)/FVC ratio (82% of predicted in mild restriction and 90% of predicted in severe restriction) but not with age (76 years in mild restriction and 69 years in severe restriction). Regardless of severity of restriction, the average DLco/VA (≥86% of predicted) remained within normal limits.CONCLUSIONS: One in four patients with IPF had normal TLC and more than one-half had a normal FVC during initial evaluation. As the severity of the restriction increased, FEV1/FVC increased, DLco decreased but DLco/VA remained normal.