Tc-99m-MDP bone scan was performed in 31 patients with primary malignant bone tumors, 22 patients with osteogenic sarcoma, 5 patients with chondrosarcoma and 4 patients with Ewing's sarcoma. The findings were classified by isotope intensity of accumulation in tumor as grade 1 to 3, overall pattern of isotope distribution in tumor as grade 1 to 3, and distortion of bony outline as grade 1 to 3. Histologic classifications were correlated with scan findings in 22 patients with osteogenic sarcoma. The results were as follows. 1) In 22 patients with osteogenic sarcoma, markedly increased isotope intensity higher than sacroiliac joint with patchy areas of decreased intensity and severe bony distortion were found in 16 patients. The correlations between histologic classification and scan findings were not discovered. 2) In 5 patients with chondrosarcoma, mildly increased isotope intensity with patchy areas of increased intensity and mild bony distortion were found in 4 patients. 3) In 4 patients with Ewing's sarcoma, markedly increased homogenous intensity with moderate bony distortion were found in 3 patients. Conclusively there were common findings in each 3 primary malignant bone tumors and Tc-99m-MDP bone scan was complemented with radiologic studies in differentiating primary malignant bone tumors.