In this study, we assessed the frequency of cyclin D1 protein expression in normal and neoplastic parathyroid tissue (10 parathyroid carcinomas, 28 adenomas, 18 hyperplasias, and 32 normal parathyroid glands) with use of a monoclonal anticyclin D1 antibody and a heat-induced epitope retrieval method. Overexpression of cyclin D1 was identified in 10 (91%) of 11 biopsy specimens from 10 patients with parathyroid carcinomas and in 11 (39%) of 28 parathyroid adenomas. In addition, 11 (61%) of 18 cases of parathyroid hyperplasia also expressed cyclin D1 protein, an observation not reported previously. These results confirm the high frequency of cyclin D1 expression in parathyroid carcinomas and adenomas. In addition, the results of this study indicate that overexpression of cyclin D1 protein is not limited to neoplastic proliferations of parathyroid tissue but is also seen in non-neoplastic proliferations of parathyroid gland. Cyclin D1 protein expression was rarely (<6%) present in normal parathyroid tissue.