Objectives: We evaluate whether integrated fluorodeoxyglucose-positron emission tomography and computed tomography (FDG-PET/CT) scan can diagnose the presence of lymph node metastasis more accurately than computed tomography (CT) scan alone. Methods: Forty-two patients with lung cancer preoperatively underwent integrated PET/CT scan using FDG and CT scan and underwent pulmonary resection and lymph node dissection. We judged cases as lymph node metastasis if the lymph node visually accumulated FDG at PET/CT scan and measured 1 cm or greater in the short axis at CT scan. We retrospectively analyzed whether our judgments in each modality were consistent with the pathological diagnosis. Results: Two-hundred and seventeen stations of lymph node were dissected and 21 stations (9.7%) were histologically diagnosed as positive metastasis. Thirty-two stations of lymph node visually accumulated FDG at PET/CT scan and 17 stations measured 1 cm or greater in the short axis at CT scan. Concerning the diagnosis of lymph node metastasis, PET/CT scan showed significantly higher sensitivity than CT scan (81% vs. 48%, p=0.024). The false-positive rate was significantly high in PET-positive lymph nodes measuring less than 1 cm in diameter. There were 4 false-negative lymph nodes with both scans. All of these were less than 7 mm in diameter and had a low percentage of metastatic foci in the lymph node. Concerning the diagnosis of N staging, there was no significant difference between PET/CT scan and CT scan (83% vs. 69%, p=0.124). However, the identification of N2 disease at PET/CT scan was significantly more accurate than that at CT scan (100% vs. 38%, p=0.031). Conclusions: PET/CT is superior to CT scan in lymph node staging. However, because the false-positive rate is high in PET-positive lymph nodes measuring less than 1 cm in diameter, we think that clinical background should be considered and other modalities or histological examinations should be undertaken if necessary. J. Med. Invest. 57: 305-313, August, 2010