PURPOSE: We studied the diagnostic significance of bone scan in evaluation of bone metastasis by lung cancer, prevalence rate, and the causes of false positive bone scan and soft tissue accumulation of bone seeking agent. MATERIALS AND METHODS: The subject included 73 lung cancer patients with bone scan. We analyzed the frequency of bone metastasis, its distribution and configuration, and any relationship between bone pain and corresponding region on bone scan. The positive findings of bone scan were compared with simple X-ray film, CT, MRI and other diagnostic modalities. The false positive bone scan and the soft tissue accumulation of bone seeking agent were analized. RESULTS: The positive findings on bone scan were noted in 26 cases(36%) and they were coexistant with bone pain in 30%. The correspondance between bone scan and bone X-ray was 38%. False positive bone scans were seen in 12 cases(16%), which include fracture due to thoracotomy and trauma, degenerative bone diseases, and bifid rib. Accumulation of bone seeking agent in soft tissue were seen in 13 cases(18%), which included primary tumor, enlarged cervical lymph node, pleural effusion, ascites and pleural thickening. CONCLUSION: Bone scans should be carefully interpreted in detecting bone metastasis in primary malignancy, because of the 16% false positivity and 18% soft tissue accumulation rate. It is very important to note that the correlation between bone pain and positive findings of bone scans was only 38%.