In 163 patients with osteosarcoma of the extremities treated with adjuvant chemotherapy, serum alkaline phosphatase (SAP) levels were evaluated before treatment and successively closely monitored in an attempt to determine whether serum levels of this enzyme had clinical value in predicting the course of the disease. Of 121 patients with elevated preoperative SAP levels, 78 (64%) recurred. Of 42 patients with normal preoperative SAP levels, only 8 (19%) recurred (P less than 0.00001). Of the 140 patients locally treated with amputation, SAP returned to normal values in 77 patients; 46 of these (59%) relapsed. Of the 21 patients whose SAP remained elevated after amputation, all but 2 relapsed (90%). Among 119 patients with normal postoperative values of SAP, at the time of relapse SAP resulted elevated in 22 of 41 patients (53%). These data confirm that in osteosarcoma, preoperative SAP levels have a definite value in establishing the prognosis and that posttreatment follow-up examination should include SAP determination, since persistent elevated or increasing values herald the appearance of a recurrence.