Pre-treatment serum lactate dehydrogenase level is an important prognostic factor in high-grade extremity osteosarcoma. Treatment of high-grade osteosarcoma remains a challenge. The prognostic significance of the pre-treatment serum lactate dehydrogenase (LDH) level is currently controversial.We reviewed records from all patients diagnosed with conventional high-grade osteosarcoma at our institution over a 25-year period and analysed the prognostic significance of LDH in high-grade localised extremity osteosarcomas treated with chemotherapy.Between June 1977 and March 2003, 66 patients for whom follow-up was available were diagnosed with localised high-grade extremity osteosarcoma and treated with chemotherapy. The median age was 15 years, with only 3% older than 40 years, and the median follow-up was 100 months. The median progression-free survival (PFS) was 67 months and the median overall survival (OS) was 113 months. The absence of a response to chemotherapy was correlated with a trend toward lower PFS and OS. High serum pre-treatment LDH level was associated in multivariate analyses with a poorer prognosis for both PFS (HR=8.623, 95%CI: 1.71-43.37; p=0.009) and for OS (HR=9.38; 95%CI: 1.73-50.74; p=0.009).In this series, the pre-treatment serum LDH level had an independent prognostic value for both PFS and OS in patients with high-grade localised extremity osteosarcoma. This measurement should be included in a large prospective prognostic series.