Nucleolar organizer region proteins, which can be stained and visualized by an argyrophil technique (AgNORs), are markers of cell activities, such as DNA transcription and proliferation, and they are useful for differential diagnosis between benign and malignant tumors. We counted both AgNOR numbers in 25 parathyroid lesions (three carcinomas, 11 adenomas, 10 hyperplasias, and one hyperplasia with carcinoma) to determine if the AgNOR number could be useful as a diagnostic aid in parathyroid neoplasms and hyperplasias, because it is often difficult to histopathologically distinguish among these lesions. The AgNOR numbers were significantly higher in carcinomas (3.18 +/- 0.05) than in adenomas (1.67 +/- 0.30, P < .001) or hyperplasias (1.85 +/- 0.16, P < .001), but there was no significant difference between adenomas and hyperplasias. These results suggest that AgNORs may be useful as an adjunct to discriminating carcinomas from adenomas or hyperplasias in the parathyroid gland.