We conducted a randomized controlled trial to determine whether intensive multi-agent adjuvant chemotherapy improves the chances of relapse-free survival in patients with nonmetastatic high-grade osteosarcoma of the extremity, as compared with concurrent controls. After undergoing definitive surgery, 36 patients were randomly assigned to adjuvant chemotherapy or to observation without adjuvant treatment. At two years the actuarial relapse-free survival was 17 percent in the control group, similar to that found in studies before 1970, and 66 percent in the adjuvant-chemotherapy group (P less than 0.001). Similar results were observed among 77 additional patients who declined to undergo randomization but who elected observation or chemotherapy. We conclude that the natural history of osteosarcoma of the extremity has remained stable over the past two decades, that adjuvant chemotherapy increases the chances of relapse-free survival of patients with high-grade osteosarcoma, and that it should be given to all such patients.