Two unusual cases of osteoblastoma with different responses to radiation therapy are reported. A 15-year-old boy with an aggressive osteoblastoma of the ilium was treated initially with curettage and subsequently with radiation and systemic chemotherapy. The patient died 35 months after his initial presentation with distant metastases which were verified at autopsy. A 12-year-old girl developed a lesion of the femur with clinical and histologic features of aggressive osteoblastoma. Amputation was recommended at the time of recurrence, but the patient declined and radiation therapy was given. The patient was alive and well 14 years after her initial presentation. These cases highlight the biologic diversity of osteoblastoma-like lesions and illustrate the difficulties which may be encountered in attempting to differentiate between benign and malignant forms of this tumor.