We present a case of non-Hodgkin's lymphoma after chemotherapy for a cancer of unknown origin. A 68-year-old man was hospitalized for hoarseness. Computed tomography(CT)scans showed a swelling of the superior mediastinal lymph node and a tumor of the right lobe of thyroid gland. Resection of the superior mediastinal lymph node and right hemithyroidectomy were performed. Pathological findings of the lymph node showed adenosquamous cell carcinoma, but no malignant lesion was found in the thyroid gland. Post-operative systemic survey failed to identify the origin of the adenosquamous cell carcinoma. Six courses of chemotherapy consisting of carboplatin and docetaxel were carried out. Seven months later, CT and positron emission tomography revealed swelling of the mediastinal lymph nodes and a tumor in the left abdominal tumor. An open biopsy of the abdominal tumor demonstrated non-Hodgkin's lymphoma, mature B cell type, follicular lymphoma, grade 1. Radiotherapy was done for the malignant lymphoma, and radiochemotherapy for the mediastinal lymph nodes. Seven months later, the patient died of systemic metastases of the adenosquamous cell carcinoma.