Case: An 18-year-old male collegiate lacrosse player presented to a sports medicine clinic with two-week history of atraumatic pain along the lateral aspect of his left arm. Radiographs demonstrated a thin, well-defined exostosis arising from the cortex of the mid-humeral diaphysis and magnetic resonance imaging demonstrated an osseous excrescence in the anterolateral aspect of the humerus and the prominence of the cortex in this area suggested a prior subperiosteal hematoma in this region -consistent with an ossified subperiosteal hematoma. Conclusion:This case reports demonstrates the first description of an ossified subperiosteal hematoma within the mid-diaphyseal region of the humerus.which allowed for the subperiosteal hemorrhage to form [7].An ossified subperiosteal hematoma of the humeral mid-diaphysis has not been described previously. As such, the purpose of this case report is to describe the clinical and radiographic features of an ossified subperiosteal hematoma of the humeral mid-diaphysis. Statement of Informed ConsentThe patient was informed that the data concerning this case would be submitted for publication and he provided consent.