Her-2/neu expression in osteosarcoma increases risk of lung metastasis and can be associated with gene amplification. The purpose of this study was to investigate whether Her-2/neu expression at diagnosis of osteosarcoma could provide biologic and prognostic information that predicts the risk of pulmonary metastases and outcome. Human epidermal growth factor (Her-2/neu) expression in 25 initial pretreatment osteosarcoma biopsies and 12 posttreatment pulmonary metastatic osteosarcoma resection specimens was assessed by standard immunohistochemical techniques on formalin-fixed paraffin-embedded tissue. As a screening analysis to determine if gene amplification may be a mechanism for increased Her-2/neu expression, FISH analysis was conducted on seven Her-2/neu immunostain-positive samples and five Her-2/neu immunostain-negative samples. Cytoplasmic Her-2/neu reactivity was identified in 11/25 (44%) of primary tumors and in 7/12 (58%) resection specimens from pulmonary metastases. Cytoplasmic Her-2/neu expression was associated with shorter overall metastasis-free survival. Her-2/neu gene amplification was identified by FISH analysis in six of the seven immunostain-positive samples but was also identified in two of the five immunostain-negative samples. Her-2/neu expression in patients with osteosarcoma is associated with an increased risk of metastasis and may define a subset of patients with a more aggressive tumor phenotype. Her-2/neu gene amplification may provide a mechanism for Her-2/neu overexpression in certain cases of osteosarcoma. Whether Her-2/neu expression influences outcome needs to be examined further in a prospective fashion. The hope is that Her-2/neu expression will identify patients who may benefit from the addition of directed biologic therapy.