Tumor size and growth are important parameters when evaluating bone and soft tissue neoplasms. There are no reports comparing the intra- and interobserver reliability among physicians in their evaluation of musculoskeletal (MSK) tumor imaging. This study investigates the accuracy and precision of measurements made by orthopedic and radiology physicians in different stages of training. Blinded magnetic resonance imaging (MRI) scans from six patients, three soft tissue, and three bone tumors were selected:​ each case included an “old” and “new” scan that was performed at least 3 months apart. Fourteen participants were selected, representing varying levels of education and experience, including two of each of the following:​ medical students, orthopedic and radiology residents, oncology and nononcologic orthopedic attendings, and MSK and non-MSK radiology attendings. Participants compared the old and new studies, recording tumor size in the transverse, cranial–caudal, and anterior–posterior dimensions, and determined if the tumor was stable or unstable. The MRI's official report served as the “gold standard.” Average intraobserver variability (|Trial 1 − Trial 2|/[(Trial 1 + Trial 2)/2])) in size measurements was 11.08% (0.00%–68.62%). The lowest variability was recorded by the MSK radiologist 1 (6.16%), and the greatest variability by Orthopedic Surgery Resident 1 (16.70%). Participants correctly determined stability 82% of the time (71%–100%). Only MSK radiologists correctly determined stability in over 90% of cases. There is considerable variability and inaccuracy in MRI-based measurements of MSK tumors. These findings motivate opportunities for improving MSK imaging education of radiology and orthopedic residents. Physicians ordering MRI scans should evaluate them themselves, instead of relying on the radiology report alone, to inform clinical decision-making.