Introduction: Standard treatment for pediatric patients with localized osteosarcoma includes high-dose methotrexate (HDMTX), and cure rates greater than 60% are observed. However, in adult patients, the toxicity profile limits the use of HDMTX and the drug is usually excluded from chemotherapy protocols for this group. We aimed to evaluate the outcomes of adult patients with localized osteosarcoma treated with chemotherapy without methotrexate. Methods: In this retrospective cohort, we evaluated adult patients with high-grade osteosarcoma who received chemotherapy treatment without methotrexate in a reference cancer center from 2007 to 2018. Outcomes analyzed were recurrence-free survival (RFS), overall survival (OS), and prognostic factors associated with overall survival. Results: A total of 48 patients had localized disease and received treatment with chemotherapy without methotrexate. The majority of them received chemotherapy with a combination of cisplatin and doxorubicin (n=42, 87.5%). Median age was 27 years (range 16.8-66.7). With a median follow-up of 29.2 months, median RFS was 29.9 months. Median OS was not reached. 5-year RFS and OS rates were 35.1% (95% CI: 20.3-50.2%) and 71.6% (95% CI: 52.3-84.2%), respectively. Patients who received cumulative doses of doxorubicin ≥375mg/m2 had better OS than those who received lower doses (HR 0.26, 95% CI: 0.07-0.94, p=0.041). Similarly, patients who received ≥6 cycles of neoadjuvant/adjuvant cisplatin tended to have better OS than those who received <6 cycles (HR 0.30, 95% CI: 0.081.09, p=0.069). Nineteen patients received less than 6 cycles of cisplatin and doxorubicin mainly because of grade 3 or 4 toxicities (11), disease progression (6), patient refusal (1), and physician choice (1). Conclusion: In our study, adult patients with localized highgrade osteosarcoma treated with chemotherapy without methotrexate had unfavorable outcomes. The cumulative doxorubicin dose and the number of cisplatin/doxorubicin cycles were associated with improved OS. The investigation of additional treatment strategies is of utmost importance to improve adult patients’ outcomes.