A 33-year-old man with a beta HCG-positive seminoma was found to have testicular intraepithelial neoplasia (TIN; carcinoma in situ) in the contralateral testis. Because of retroperitoneal bulky disease (stage IIc), the patient received single agent carboplatin therapy. Complete remission, as judged radiologically and serologically, was achieved after three cycles, but the control biopsy of the testis revealed persistent TIN. Only after local radiotherapy with 20 Gy to the afflicted testis did TIN disappear. Chemotherapy is not safe for the treatment of TIN, and it usually cannot prevent contralateral testicular tumors.