Only very few previously reported cases of pronounced lymphocytic infiltration in parathyroid adenoma can be found in the English medical literature. The objective of this report is to present such a rare case and to investigate to a certain extent the immunohistochemical profile of this rare histologic observation. The lymphoid cell population within the tumour was composed of nodule-forming B-cells and different subsets of infiltrating T-cells and caused minimal destruction of neoplastic tissue.