The cause of idiopathic pulmonary fibrosis (IPF) remains unknown. Although the observed biologic and biochemical processes associated with the disease are consistent with a fibrotic process, they are not necessarily unique to IPF. Furthermore, the importance of these observations will not be apparent until a directed therapy alters the natural history of the disease. There are essentially no studies that explain the unique histologic features of this disease. As mechanistic data accumulates, it is our opinion that these data should pass the test of explaining the clinical histologic features of the disease before it can be assumed that these features are unique for IPF.