We evaluated the rate of second malignancies in 1205 patients with osteosarcoma of the extremity treated at our Institution with different protocols of adjuvant and neoadjuvant chemotherapy. Twenty-six patients (2.15%) developed a second malignant neoplasm at a median of 7.6 years (1 to 25 y) after primary osteosarcoma treatment. Of these, 2 developed a third cancer which were not considered in the series. Second neoplasms were leukemia (10), breast (7), lung (2), kidney (2), central nervous system cancer (2), soft tissue (1), parotid (1), and colon (1). The rate of second neoplasms was significantly higher in female patients, and the latent period shorter in hematologic tumors compared with solid tumors. Ten of these 26 patients are disease free at a median of 7.7 years (range 1 to 15 y) after the last treatment. The rate of second malignancies observed in the osteosarcoma group was significantly higher than that observed in the control group of 1160 patients with benign tumors treated in the same period at our Institute (2.2% vs. 0.8%, P<0.009). Our study showed that the risk of second neoplasm within 15 years increased and then leveled off and that although secondary solid tumors could be explained as unrelated cases, leukemias seem to be over represented.