The pulmonary parenchyma in interstitial fibrosis responds stereotypically to a variety of insults. The observation that only a small subset of persons exposed to known fibrogenic agents develop fibrosis and the induction of pulmonary fibrosis in genetically susceptible mice support a genetic predisposition. The description of familial idiopathic pulmonary fibrosis is the strongest evidence available supporting this hypothesis. Unfortunately, despite all the recent advances in molecular genetic techniques there have been few human studies to date. Given the poor prognosis and the lack of a cure for pulmonary fibrosis, future genetic studies will, it is hoped, clarify the pathogenesis of pulmonary fibrosis and lead to preventive measures and new therapeutic interventions. Idiopathic pulmonary fibrosis typically occurs in patients who are older than 50. The etiology is unknown.