Abstract BACKGROUND CPC are childhood cancers in which the loss of TP53 function defines unfavorable disease. CPC-free survival rates have remained poor in patients with TP53 mutations and recent data revealed CPC-free survival of 29% with a non-marrow-ablative chemotherapy strategy. Previously reported results of the “Head Start” (HS) HDCT approach were limited by small patient numbers and incomplete TP53 mutation data. Consequently, the role of HDCT in TP53-mutated CPC remains unclear. METHODS We have analyzed data on TP53 status, CPC-free survival and second cancers from the largest cohort of patients who received initial therapy with the intent of undergoing HDCT. RESULTS Twenty-seven children with CPC received HS-like induction chemotherapy and 23 completed HDCT consolidation. Somatic or germline TP53 mutations were identified in 17 patients, 4 exhibited TP53 wild type (TP53wt); TP53 status remained untested in 6. Five-year CPC-free survival was 66% for patients with TP53 germline mutations (n=12), 40% for those with somatic tumor TP53 mutations (n=5) and 50% with confirmed TP53wt (n=4). Among patients with germline TP53 mutations, only 4/12 experienced CPC recurrences, 4 died from secondary cancers and 4 are currently alive. The 5-year CPC-free survival for 23 patients who underwent HDCT was 52%. All six survivors with TP53 mutations had undergone HDCT, and 5/6 avoided irradiation. Eight/12 patients with TP53 germline mutations and Li-Fraumeni Syndrome (LFS) developed second malignancies, including 6 patients in HDCT group and the remaining 2 patients who did not undergo consolidative HDCT. CONCLUSIONS Our data indicate that HDCT consolidation is associated with improved CPC-free survival compared with historical reports in children with TP53 mutated CPC. The ultimate development of second cancers affects outcomes for those with LFS, irrespective of HDCT treatment. These data justify the inclusion of HDCT in the prospective international trial under development for young children with newly diagnosed TP53 mutated CPC.