The objective of this study was to assess the utility of C-reactive protein (CRP) on differential diagnosis of pulmonary infiltrates occurring in 143 febrile patients with hematologic malignancies. Serum CRP level, measured on the first day of pneumonia, was significantly higher in patients with fungal lung infiltrates than in those with nonfungal pneumonia (22.3 mg/dl vs 7.3 mg/dl; p<0.0001). Predictive factors for fungal pneumonia were CRP level higher than 10 mg/dl, neutropenia longer than 10 days, and active underlying disease.