The usefulness of a new method to detect B-cell monoclonality named VR, which is a combination of the digestion with restriction enzymes and vectorette PCR, in aspiration biopsy-nucleic acid diagnosis (ABND) of thyroid malignant lymphoma was investigated. ABND was performed in 12 patients in whom malignant lymphoma was suspected. Among the patients who underwent open surgery, monoclonality was detected in the aspirates in four (50%) of eight patients with malignant lymphoma, but was not detected in one patient with Hashimoto disease. ABND using VR is a useful adjunct of aspiration biopsy cytology in the diagnosis of thyroid malignant lymphoma.