The purpose of this study is to determine whether NAC added to prednisone, and azathioprine has a better effect on lung function, radiology and clinical condition than placebo + prednisone in combination with azathioprine after 6 and 12 months.

Inclusion Criteria:

• Diagnosis of IPF according to the International Consensus Statement
• Bronchoalveolar lavage (BAL) showing no features to support an alternative diagnosis.
• Patients in whom it was possible to determine single breath DLco.
• Patients with newly or previously diagnosed IPF, in whom it was clinically justified to use the standardised regimen azathioprine plus prednisone

Exclusion Criteria:

• Known intolerance to N-Acetylcysteine.
• Patients with respiratory infections at study entry should be excluded until the infections have been treated successfully (VC and Dlco comparable with the values before the infection).
• Patients with pre-existing disease that interferes with the evaluation of IPF: extensive old TBC lesions, significant bronchiectasis, moderate or severe COPD.
• Patients with malignancy in the last 5 years. If the patient had a malignancy in the past and is free of malignancy for more than five years, the patient is regarded as healed.
• Patients with heart failure.
• Patients with hepatic function abnormalities contraindicating the use of azathioprine (i.e. clinically significant abnormalities of PTT and/or GGT).
• Patients with a renal clearance < 10ml/min and/or hematuria and/or proteinuria of collagen vascular disease origin. A renal clearance is only performed in the presence of an abnormal serum creatinine and/or serum urea level.
• Patients who are artificially ventilated.
• Prednisone at a dose > 0.5 mg/kg/day (or other glucocorticoids such as triamcinolone, dexamethasone or methylprednisolone at an equivalent dose) or azathioprine at a dose > 2 mg/kg/day during the last month prior to inclusion.
• Use of other immunosuppressives (such as cyclophosphamide and colchicine) is not allowed in the last month and for the duration of the trial.
• Any form of anticancer therapy or methotrexate are not allowed when used for more than 1 week in the past and for the duration of the trial.
• Amiodarone or nitrofurantoin are not allowed in the last 5 years, when used for more than 1 week in the past and for the duration of the trial.
• Allopurinol, oxypurinol, thiopurinol, anti-oxidants (e.g. vitamin E) or glutathione supplements are not allowed in the last month and during the trial.
• The use of interferon or other antifibrotics (e.g. pirfenidone) is not allowed in the past and during the study.
• The use of NAC therapy at a dosage of more than 600 mg/day is not allowed in the last 3 years for a total period of more than 3 months.
• Patients suffering or having suffered from documented active ulcer within the last 3 years.
• Patients in whom the standardised treatment regimen is contraindicated or not justified.
• Pregnancy.
• Known or suspected drug or alcohol abuse.
• Patients on other investigational compounds or participating in clinical trials on investigational compounds within the last 3 months.
• Patients expected to be non-compliant in taking the medication.