In a study of 20 primary tumors of osseous pelvis proven pathologically computed tomographic appearance was analysed and correlated with the conventional radiographic appearance in terms of destructive patterns trmor-bone margins internal architectures periosteal reaction and extents of the tumors. CT was invaluable for the assessment of the extent of tumors/associated extraosseous soft tissue masses and the detection of the internal matrix within the tumor. We found the additional information about the ex-tent of soft tissue mase in 4 cases(20%) and about the internal matrix in 5 cases (25%) . According to the analysis of the patterns of cortical transgression grade III was absent in benignancy but in 6 cases of malignant tumors CT is less useful in the evaluation of the periosteal reaction and tumor-bone margin in flat bony lesions not tumor-soft tissue margins. Certain types of tumor behavior characterizing the lesions under discussion may be helpful in diagnoisis.