PURPOSE: To evaluate prospectively relationship between the extent of abnornelities of fibrosing on high resolution CT and these of pulmonary function test, and bronchoalveolar lavage. MATERIAL AND METHODS: This study included 27 patients with pathologically proved UIP. We prospectivel analyzed correlation of the extent of gound glass opacity, consolidation, irregular lines with honeycombing initial CT scan and quantitative change of ground glass opacity on follow up CT scans with pulmonary function test(PFT) and bronchoalveolar lavage(BAL). RESULTS: On initial CT scan, score of extent of disease significantly correlated with FEV1(r=-0.5148, p=0. 0310) and DLco(r=--0.7102, p=0.0001). Proportion of indivisual cell in BAL was not correlated with ground glass opacity or extent of disease;lymphocyte(p=-0.0165, r=0.9616), eosinophil(p=-0.0077, r=0.9832), neutrophil: (p=-0.1732, r=0.5903). On follow up CT scan, quantitative change of ground glass opacity correlated with DLco/VA(r=-0.6825, p=0. 0025) and TLC(r=0.5248, p=0.0429). CONCLUSION: The quantitative analysis of extent of disease on HRCT is useful for evaluation of clinical statu& Also change of ground glass opacity on follow-up HRCT significantly correlated with improvement of diffusing capacity in fibrosing alveolitis.