Review of magnetic resonance imaging was compared with the pathologic review of reexcision specimens from 24 pediatric patients who had initial incomplete resection for soft tissue sarcoma to determine the accuracy of magnetic resonance imaging in detecting residual tumor. The median age of patients was 12.8 years. The most common diagnosis was synovial sarcoma (eight of 24 patients; 30%). Magnetic resonance imaging detected residual tumor in eight patients and no tumor in eight patients; the images of eight patients were indeterminate. Pathologic examination showed residual tumor in 14 of 24 (58%) specimens obtained in reexcision. For two of the eight patients (25%) for whom imaging had detected no tumor, microscopic examination detected residual tumor; for one of the eight patients for whom imaging had detected tumor, microscopic examination detected no tumor. Five of the eight patients (63%) for whom the results of imaging studies were indeterminate had residual tumor on microscopic evaluation. The sensitivity of magnetic resonance imaging for detecting residual tumor was 78%, the specificity was 86%, the positive predictive value was 0.78, and the negative predictive value was 0.86. Decisions regarding the need for additional resection should not be based on magnetic resonance imaging alone.