In 560 patients with high-grade osteosarcoma of the extremity treated with 5 different protocols of neoadjuvant chemotherapy at a single institution between 1983 and 1995, the pre-treatment serum alkaline phosphatase (SAP) was examined to evaluate whether the enzyme levels had a clinical value in predicting the course of the disease. SAP was normal in 302 (54%) patients and high in 258 (46%). High levels of SAP was observed significantly and independently more frequently in male patients over 14-years-old, and in tumours larger than 150 ml and of osteoblastic subtypes. The 5-year event-free survival (EFS) and overall survival (OS) for all patients were respectively 60 and 68%. With multivariate analysis only two factors were independently correlated with the 5-year EFS: SAP levels (p=0.002) and the grade of chemotherapy-induced necrosis (p=0.0001). The authors conclude that in planning randomized clinical trials of neoadjuvant treatment for osteosarcoma, patients should be stratified according to SAP levels, and that when tailoring the aggressiveness of postoperative chemotherapy to the risk of relapse, in addition to the histologic response to preoperative treatment, the SAP levels should also be considered.