Back pain is a common complaint in the outpatient setting. The etiology is most often benign but it can be a serious, even life-threatening problem. This report describes a 33-year-old Caucasian male who presented with severe upper back pain for three weeks that did not respond to symptomatic outpatient treatment. Imaging studies revealed a mediastinal mass and lymphadenopathy with superior vena cava and tracheal compression. Pathology showed a poorly-differentiated malignant neoplasm consistent with seminoma. No evidence of primary testicular tumor was found. His atypical presentation of back pain was thus consistent with an extragonadal seminoma in the mediastinum.