Open lung biopsy (OLB) provides a safe, reliable method of obtaining a histological diagnosis in patients with chronic, stable lung disease. In these patients OLB is indicated where other closed methods have failed or as the primary diagnostic tool when interstitial lung disease is suspected on conventional chest X-ray or from pulmonary function data. In the critically ill patient in whom the establishment of a treatable diagnosis is imperative OLB provides the highest likelihood of obtaining a diagnosis. This must be weighed against the increased risks following OLB in this group of patients by the clinician in each case.