A 71-year-old Japanese female Nagasaki-atomic-bomb survivor was admitted for evaluation of a mediastinal mass. She was infected with human T-cell leukemia virus type I. Histological examination of the biopsy specimen, obtained thoracoscopically from the mass, revealed amyloid lymphadenopathy confirmed by Congo-red staining and electron-microscopic examination. Amyloid deposits and the serum monoclonal peak consisted of immunoglobulin lambda light chains. No atypical cells were detected in bone marrow. The case was diagnosed as an unusual amyloidosis affecting the mediastinal lymph node. No other sites were found to be affected by amyloidosis, although systemic involvement could not be conclusively ruled out.