Thymic lymphoblastic lymphoma and myasthenia gravis rarely coexist. Only two cases have been reported and we describe here a third case. A 60 year old man presented a typical history of myasthenia gravis, confirmed by neurological investigations including electromyography. Chest X-ray revealed an anterior mediastinal tumor. At thoracotomy, a 60 mm mass adherent to the pericardium was excised and a lymphoblastic lymphoma was diagnosed. The lymphogram showed enlarged pelvic and abdominal lymph nodes consistent with lymphoma. A m-BACOD chemotherapy regimen gave rapid and complete remission of both lymphoma and myasthenia gravis and the patient is now alive 25 months after the start of chemotherapy with no evidence of disease.