Idiopathic pulmonary fibrosis (IPF) is a chronic disorder that is associated with a poorer prognosis than subacute idiopathic interstitial pneumonias (IIPs). IPF can be differentiated from other IIPs on the basis of its histologic pattern of usual interstitial pneumonia (UIP). Although a surgical lung biopsy specimen showing a UIP pattern is required for the definitive diagnosis of IPF, clinical criteria can be used in the absence of a lung biopsy specimen to make a likely diagnosis of IPF. The predictive value of these criteria largely depends on the expertise of the clinician and radiologist, but considerable interobserver variability exists even when evaluations are performed by experts in the field. Moreover, these criteria lead to misdiagnosis in about 25 to 35% of cases. Interobserver variability is reduced and diagnostic accuracy is improved in cases in which a diagnosis is made with a high degree of confidence. Diagnostic accuracy is also higher when the diagnosis is made by a core group of experts rather than by a referring center. The decision on whether or not to perform a surgical lung biopsy is difficult. It is clearly indicated in cases in which clinical or radiologic findings are atypical or when the diagnosis is made with a low degree of certainty.