In a study of 20 primary tumors of osseous pelvis proven pathologically, computed tomographic appearance wasanalysed and correlated with the conventional radiographic appearance in terms of destructive patterns, tumor-bonemargins, internal architectures, periosteal reaction and extents of the tumors. CT was invaluable for theassessment of the extent of tumors/associated extraosseous soft tissue masses and the detection of the internalmatrix within the tumor. We found the additional information about the extent 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 corticaltransgression, grade III was absent in benignancy, but in 6 cases of malignant tumors. CT is less useful in theevaluation 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 diagnosis.