Bone scans with 99mTc-methylene diphosphonate were analysed in 44 patients with biopsy proven malignant bone tumor who admitted Department of Orthopedic Surgery in Seoul National University Hospital from March 1979 to June 1981. The results obtained were as follows: 1. Cases, there were 21 cases of primary malignant bone tumors and 23 cases of metastatic bone tumors. 2. Bone scan showed single lesion in 18 cases (86%) of 21 primery malignant bone tumors, and mutiple lesions in 15 csses (65%) of 23 metastatic bone tumors. 3. There were 2 cases of cold area in bone scan of a reticulum cell sarcoma and a metastatic adenocarcinoma. 4. In comparison of bone scan and roentgenogram by number of lesions, 96 lesions were positive in bone scan or roentgenogram in 44 cases. In 21 primary maligmant bone tumors of 29 lesions, 24 lesions (83%) were positive in both bone scan and roentgenograms. In 23 metaatatic bone tumors of 67 lesions, 28 lesions (54%) were positive in both bone scan and roentgenogram, 40 lesions (42%) were positive in bone scan and negative in roentgenogram 5. In only 2 cases of multiple myeloma, showes positive in roentgenogram but normal in bone scan. 6. In metastatic bone tumor, bone scan showed 39 hot lesions without clinical symptoms as well as 27 lesions with clinical symptoms.