Testicular neoplasms occur in more than 90% of cases, due to primary testicular germ cell tumors. Other entities are non germ cell tumors of the testis, testicular manifestation of lymphomas or metastases. International and interdisciplinary co-operation has led to the development of urological guidelines and to good therapeutic success for testicular neoplasms. The gold standard for treatment of a testicular neoplasm is the radical orchiectomy. However, for individual cases with suspected lymphoma, a treatment decision differing from the guidelines may be reasonable. We present the case of a 38-year-old man with testicular manifestation of a transformed mycosis fungoides, which is the most common form of cutaneous T-cell lymphoma.