We report a case of primary mediastinal seminoma, which presented initially with shortness of breath and a swelling in upper part of anterior chest wall. The diagnosis of primary mediastinal seminoma was established on the basis of histologic findings and was confirmed by immunohistochemical analysis. Abdominal, pelvis and cerebral CT scan, testicular ultrasound and TC-99 MDP bone scintigraphy were negative. Chemotherapy was initiated with B.E.P. protocol (Bleomycin, Etoposide, Cisplatinum); the patient received four cycles of chemotherapy. After 8 months, the patient was seen in the clinic; he was well.