Purpose : To observe sequential changes of acute and subacute hypersensitivity pneumonitis in high resolutionCT and to correlate the findibgs 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 subacutehypersensitivity pneumonitis. The extent of ground glass attenuation and nodules on high resolution CT scan wascorrelated with pulmonary function test and bronchoalveolar lavage. We also evaluated serial changes of the lesionin high resolution CT scans. Results : The extent of parenchymal abnormalities on high-resolution CT scans weresignificantly correlated with diffusing capacity (GGA & DLCO r=-0.95 p < 0.003, Nodule & DLco r=-0.94 P < 0.005 )and FEV1 ( GGA & FEV1 r=-0.57, p < 0.05, Nodule & FEV1 r=-0.56 P < 0.05 ) on pulmonary function test andrelatively correlated with total count of cells (GGA & total count of cells r=0.86 P < 0.03, Nodule & total countof cells r=0.71 p < 0.11 ) on bronchoalveolar lavage. The order of disappearance of abnormal findings were poorlydefined centrilobular nodule, ground glass attenuation, and well defined small centrilobular nodule on sequentialCT scans. Conclusion : The authors conclude that HRCT is useful for diagnosis and follow up evaluation of theacute and subacute hypersensitivity pneumonitis. Quantitative analysis of extent of disease on HRCT is useful forevaluation of clinical status.