A 78-year-old man presented with swelling of para-aortic and left iliac lymph nodes (LNs). He had undergone left high orchiectomy 10 years ago. He was diagnosed with stage I seminoma, and was managed with active surveillance. Eight years later, the follow-up computed tomography (CT) revealed para-aortic LN swelling, but the patient refused further treatment. He complained of left lower extremity edema 10 years after orchiectomy. CT showed enlargement of both LNs, especially, diameter of left iliac lymph nodes was up to 9 cm. He was referred to our hospital. LDH was slightly increased to 261 IU/1, but α-fetoprotein, and total and free beta human chorionic gonadotropin were within normal limits. Results of pathological review of the testicular tumor was also seminoma. The treatment with etoposide and cisplatin (EP) was started under the diagnosis of late relapse of seminoma. However, CT after 1 course of EP showed no shrinkage of LN metastases. At this time, soluble interleukin-2 receptor (sIL-2R) was elevated up to 5,090 U/ml (normal range: 613 U/ml). Needle biopsy from pelvic LN was performed on suspicion of metachronous malignant lymphoma. The pathological diagnosis was mantle cell lymphoma. The patient was transferred to the department of hematology, and treated successfully with rituximab and bendamustine. When LN swelling is seen after long-term active surveillance, there is a possibility of late relapse. However, metachronous malignant lymphoma also should be considered in an elderly patient.