Objective: Diffuse interstitial lung disease (DILD) is a group of diverse diseases that share common clinical, radiologic, and pulomonary function features. Open lung biopsy (OLB) has been regarded as gold standard in differential diagnosis of DILD. However open lung biopsy is a invasive diagnostic tool not free of its own risk or complications. These days, high-resolution computed tomography (HRCT) has become an important diagnostic tool in DILD through its precise image analysis. In many instances, HRCT could provide specific diagnosis or, at least, provide information on the disease activity of DILD. The authors re-evaluate the role of open lung biopsy in this "HRCT era" by investigating the additional diagnostic gain and impacts on the treatment plan in patients who have undergone high-resoluticm CT. Method: Diagnoses obtained by high-resolution CT and open lung biopsy were compared and changes of treatment plans were evaluated retrospectively in 30 patients who had undergone open lung biopsy for the purpose of diagnosis of diffuse interstitial lung disease from March 1988 to June 1994. Results: High-resolution CT suggeted specific diagnoses in 22 out of 28 patients (78.6%) and the diagnoses were confirmed to be correct by open lung biopy in 20 of those 22 cases (91%). Open lung biopsy could not give specific diagnosis in 5 out of 30 cases (16.7%). In 5 out of 6 cases (83.3%) in whom high reolution CT was not able to suggest specific diagnosis, open lung biopsy gave specific diagnoses. Treatment plan was altered by the result of open lung biopsy in only 2 cases. Conclusion: The above findings suggest that in "HRCT era", when HRCT could suggest specific diagnosis, the need for open lung biopsy should be re-evaluated.