['To analyze the significance of relative tumor burden in pediatric osteosarcoma.', 'We retrospectively reviewed 241 patients with localized pediatric osteosarcoma and measured tumor size on MR images at presentation. Absolute tumor size parameters measured on MR images were converted to relative values by dividing body surface area. Both absolute and relative size parameters were analyzed for their correlation with metastasis-free survival (MFS).', 'The 5-year MFS was 67.5 +/- 3.1%, with median follow-up of 72 months (range, 8-205 months). In addition to poor histologic response (RR 3.24; 95% CI: 1.99-5.28; P < 0.001), large relative tumor plane (RTP; RR 3.28; 95% CI: 1.72-6.25; P < 0.001), large relative tumor area (RTA; RR 4.14; 95% CI: 1.53-11.22; P = 0.005), and large absolute tumor width (ATW; RR; 3.10; 95% CI: 1.23-7.79; P = 0.02) shortened the MFS. When survival was analyzed by combining RTP and histologic response, patients with small RTP and a good response showed the best survival (5-year MFS of 90.2 +/- 3.6%), while those with large RTP and a poor response showed the worst survival (5-year MFS of 45.1 +/- 6.6%).'] Does relative tumor burden predict metastasis-free survival in pediatric osteosarcoma?