& We retrospectively reviewed our experience with ® ne needle aspiration biopsy (FNAB) in the diagnosis and management of skeletal osteosarcoma. The bi-institutional study sample involved 30 consecutive aspirates from 29 patients (28 primary tumors, 1 pulmonary metastasis, 1 local recurrence). There were 17 children and 12 adults. Two aspirates were unsatisfactory for diagnosis. Of the adequate primary osteosarcomacases analyzed by FNAB, 24 of 26 were diagnosedas osteosarcoma.All pediatriccases were correctly interpreted as osteosarcoma and treated appropriately. There were 2 incomplete diagnoses. A secondaryosteosarcomaarising within an otherwise clinically, radiologically,and histologically typical giant cell tumor (malignant giant cell tumor) was not diagnosed preoperatively on FNAB due to nonrepresentative sampling. Chronologically, the ® rst patient with osteosarcoma analyzed by FNAB was diagnosed simply as``spindle cell neoplasm.' ' No complications resulted from the procedure. With adequate clinical and radiologic correlation, FNAB represents a technically, easily performed, cost-eÚ ective, and accurate procedure for establishing the diagnosis of skeletal osteosarcoma. Immediate interpretation of aspirated material allows for therapy planning and oncologic consultation at the initial clinic visit.