Pirfenidone to Prevent Fibrosis in Ards.

Inclusion Criteria: Concomitant presence of: * ARDS (moderate and severe) - Berlin definition 1. Within 1 week of a known clinical insult or new or worsening respiratory symptoms 2. Bilateral opacities on CXR which are not fully explained by effusions, lobar/lung collapse or nodules 3. Respiratory failure not fully explained by cardiac failure or fluid overload 4. PaO2/FiO2\<200 mmHg with PEEP\<=5 cmH2O (invasive mechanical ventilation) * Inflammatory ARDS phenotype (28), defined by at least one of the following: 1. High plasma levels of inflammatory biomarkers 2. Vasopressor dependence 3. Lower serum bicarbonate or increased serum lactate * Informed consent expressed by the patient or by legal representative or on the Ethical Committee indication. * Age \>=18 years Exclusion Criteria: * Intubated and mechanically ventilated via an endotracheal or tracheostomy tube (\>7 days) up to the time of randomization * ARDS severe or moderate for more than 36 hours * Untreated pulmonary embolism, pleural effusion or pneumothorax as the primary cause of ARF * ARF fully explained by left ventricular failure or fluid overload * Consent declined * Severe chronic respiratory disease requiring domiciliary ventilation * Clinical suspicion for significant restrictive lung disease * Pregnant women or women of childbearing potential who are sexually active * Known allergy to pirfenidone * Concomitant use of fluvoxamine * Known severe hepatic failure * Known severe renal failure or necessity of dialysis not related to acute disease * Little chance of survival (SAPS II score\>75)