Purpose: Incidence of lung cancer in patients with idiopathic pulmonary fibrosis (IPF) is known to be higher than that in general population. However, it is difficult to discriminate pulmonary nodule in patients with IPF, because underlying IPF can be expressed as lung nodules. We evaluated the diagnostic performance of FDG PET in discriminating lung nodule in patients with IPF. Methods: We retrospectively reviewed 28 lung nodules in 16 subjects (age; 67.53¡¾9.83, M:F=14:2). Two patients had previous history of malignant cancer (small cell lung cancer and subglottic cancer). The diagnostic criteria on chest CT were size, morphology and serial changes of size. FDG PET was visually interpreted, and maximal SUV was calculated for quantitative analysis. Results: From 28 nodules, 18 nodules were interpreted as benign nodules, 10 nodules as malignant nodules by histopahthology or follow-up chest CT. The sensitivity and specificity of FDG PET were 100% and 94.4%, while those of CT were 70.0% and 44.4%, respectively. Malignant nodule was higher maxSUV than that of benign lung nodules (7.68¡¾3.96 vs. 1.22¡¾0.65, p<0.001). Inflammatory lesion in underlying IPF was significantly lower maxSUV than that of malignant nodules (1.80¡¾0.43, p<0.001). The size of malignant and benign nodule were 23.95¡¾10.15 mm and 10.83 ¡¾5.23 mm (p<0.01). Conclusion: FDG PET showed superior diagnostic performance to chest CT in differentiating lung nodules in patients with underlying IPF. FDG PET could be used to evaluate suspicious malignant lung nodule detected by chest in patients with IPF. (Nucl Med Mol Imaging 2006;40(3):163-168)