To find prognostic factors for intensive care unit (ICU) days in patients with acute eosinophilic pneumonia (AEP). Method: Consecutive patients who were diagnosed as AEP were enrolled. Clinical parameters of patients were collected. The diagnosis and treatment was according to standardized protocol. Result: From May 2007 to October 2010, 138 patients were diagnosed. Ninety-eight percent of patients were smoking related. Mean age was 20.21±0.09 (mean±SEM) years. The initial PaO2/FiO2 (PF) ratio was 281.94±6.22. Peripheral blood (PB) eosinophil was 4.37±0.39 % and bronchoalveolar lavage (BAL) eosinophil was 44.98±1.23 %. C-reactive protein (CRP) level was 9.64±0.46 mg/dL. Radiologic findings were ground glass opacity (97.1 %), air-space consolidation (37.0 %), poorly defined centrilobular nodules (51.4 %), interlobular septal thickening (68.1 %), thickening of bronchovascular bundle (17.4 %), and pleural effusion (88.4 %). One hundred nine (79.0 %) patients were admitted to the ICU, and the mean duration of ICU stay was 2.50±0.13 days. Correlation between ICU days and other clinical parameters were examined. ICU days were significantly correlated with the PF ratio (r=-0.287, p=0.002) and CRP (r=0.361, p<0.0001). However other clinical parameters (BAL eosinophil, PB eosinophil, and age) were not correlated with ICU days. Also, there was no significant difference in ICU days according to radiologic findings. In a multivariable linear logistic regression analysis, PF ratio and CRP level were significant prognostic factors to determine ICU days. Conclusion: PF ratio and CRP were prognostic factors for predicting ICU days in patients with AEP. Further study about the prognostic factor in patients with AEP is needed.