Pirfenidone is an orally active small molecule that has recently been evaluated in large clinical trials for the treatment of idiopathic pulmonary fibrosis, a fatal disease in which the uncontrolled deposition of extracellular matrix leads to progressive loss of lung function. This review describes the activity of pirfenidone in several well-characterised animal models of fibrosis in the lung, liver, heart and kidney. In these studies, treatment-related reductions in fibrosis are associated with modulation of cytokines and growth factors, with the most commonly reported effect being reduction of transforming growth factor-b. The consistent antifibrotic activity of pirfenidone in a broad array of animal models provides a strong preclinical rationale for the clinical characterisation of pirfenidone in pulmonary fibrosis and, potentially, other conditions with a significant fibrotic component. KEYWORDS: Animal model, antifibrotic, fibrosis, idiopathic pulmonary fibrosis, pirfenidone F ibrosis, the dysregulated deposition of extracellular matrix (ECM) with progressive destruction of normal tissue, is a primary or contributing factor in chronic disease states in several organs. Pulmonary fibrosis is associated with numerous diffuse parenchymal lung diseases, of which the most common are idiopathic pulmonary fibrosis (IPF) and sarcoidosis [1]. Cardiac fibrosis is associated with chronic heart failure, atrial fibrillation, and cardiac remodelling following acute myocardial infarction [2,3]. Renal fibrosis is associated with multiple forms of chronic kidney disease and correlates with impairment of kidney function [4,5]. Hepatic fibrosis is associated with chronic hepatitis B and C viral infections and nonalcoholic steatohepatitis [6]. Each of these conditions represents a significant unmet medical need, warranting significant research and clinical study into treatment for fibrotic disease.Pirfenidone (Esbriet1, Pirespa1) is an orally active small molecule comprising a modified phenyl pyridone ( fig. 1). The compound exhibits well documented antifibrotic and anti-inflammatory activities in a variety of animal and cellbased models, although its molecular target has not been elucidated. Pirfenidone was initially identified as having anti-inflammatory activity in animal models and evaluated for use as an antiinflammatory drug [7,8]. However, the unexpected identification of antifibrotic effects in animals treated with pirfenidone redefined the interest in the compound [9]. Subsequently, pirfenidone has been shown to attenuate fibrosis in numerous animal models, including fibrosis of the lung, liver, heart and kidney.The most extensive clinical studies of pirfenidone are for treatment of IPF, a chronic interstitial lung disease characterised by the unregulated deposition of ECM leading to the unremitting destruction of normal lung. Patients diagnosed with IPF typically experience progressive pulmonary insufficiency, and most die of respiratory failure. The estimated median survival upon diagnosis is approximately ...