Ephrin A4 expression in osteosarcoma, impact on prognosis, and patient outcome. Ephrin A4 is one of the ephrin ligand molecules belonging to the tyrosine kinases receptor family. It was originally identified in a T-lymphoma cell line and seen to be expressed in human adult tissue as well as several tumor types. In our previous study, we showed the unique pattern of ephrin A4 immunohistochemical staining, which differed according to the type of examined bone specimens (normal bone, primary, and metastatic osteosarcoma lesions). The aim of the present study is to evaluate the prognostic impact of ephrin A4 expression in a group of primary osteosarcoma patients.Ephrin A4 immunohistochemical expression was carried out on 47 primary osteosarcoma cases.Ephrin A4 was expressed in 82.9% of osteosarcoma cases with cytoplasmic localization in 58.9% of positive cases. The cytoplasmic pattern was significantly associated with aggressive histopathological types of osteosarcoma (P = 0.02), advanced stage (P = 0.04), the presence of metastasis (P = 0.03), inferior response to neoadjuvent chemotherapy (P = 0.04), and tended to be associated with a shorter event-free survival (P = 0.09).The cytoplasmic pattern of ephrin A4 could identify a subgroup of primary osteosarcoma patients with a high liability for progression, poor prognosis, and inferior response to chemotherapy.