We present a case of a 52-year-old man with myasthenia gravis and a mediastinal tumor who was admitted to our hospital for surgical treatment. The pathologic examination of the resected tumor revealed a very rare case of a collision tumor: a B1B2 thymoma and a small lymphocytic lymphoma. Flow cytometry of the peripheral blood revealed the presence of a small number of leukemic cells. After postoperative irradiation of the mediastinum and chemotherapy a complete response in both diseases was achieved. The case confirms that a pathologist should always be aware that two different neoplasms can coexist in rare cases.