PURPOSE: To observe sequential changes of acute and subacute hypersensitivity pneumonitis in high resolution CT and to correlate the findings with pulmonary function test and bronchoalveolar lavage. MATERIALS AND METHODS: This study includes 11 patients with pathologically (n=10) and clinically(n=1) proved acute and subacute hypersensitivity pneumonitis. The extent of ground glass attenuation and nodules on high resolution CT scan was correlated with pulmonary function test and bronchoalveolar lavage. We also evaluated serial changes of the lesion in high resolution CT scans. RESULTS: The extent of parenchymal abnormalities on high-resolution CT scans were significantly correlated with diffusing capacity (GGA & DLco:r=0.95, p<0.003, Nodule & DLco:r=-0.94, P<.005) and FEV1 (GGA & FEV1: r=-0.57, p<.05, Nodule & FEV1: r=-0.56, P<.05) on pulmonary function test and relatively correlated with total count of cells (GGA & total count of cells: r=0.86, P<.03, Nodule & total count of cells: r=0.71, p<0. 11) on bronchoalveolar lavage. The order in disappearance of abnormal findings were poorly defined centrilobular nodule, ground glass attenuation, and well defined small centrilobular nodule on sequential CT scans. CONCLUSION: The authors conclude that HRCT is useful for diagnosis and follow up evaluation of the acute and subacute hypersensitivity pneumonitis. Quantitative analysis of extent of disease on HRCT is useful for evaluation of clinical status.