The accuracy of preoperative computed tomography (CT) and magnetic resonance (MR) imaging in the measurement of intraosseous tumor extent in 17 cases of osteosarcoma of an extremity was compared with macroslides of surgical specimens. Longitudinal intraosseous extension of tumor from the adjacent articular surface was measured on imaging studies and macroslides to the nearest millimeter. The average difference between macroslide and CT measurements (15 cases) was 16.5 mm +/- 10.7, and the average difference between macroslide and MR imaging measurements (17 cases) was 4.9 mm +/- 4.3. However, much of the latter difference appears to be caused by use of a different plane of section in MR imaging compared with that in macroslides, since in a subgroup of five specimens with an identical plane of section, the average difference was 1.8 mm +/- 1.6. The authors conclude that MR imaging is extremely accurate in the assessment of intraosseous extent of osteosarcoma.