Parathyroid adenoma with prominent lymphocytic infiltrates is very rare, with only 8 previously reported cases in the English scientific literature. Cases with prominent lymphocytic infiltrates of ectopic (mediastinal) parathyroid adenomas have not been reported. The prominent lymphocytic infiltration may make the diagnosis difficult on frozen sections, even on permanent histologic sections. We herein describe a case of ectopic (mediastinal) parathyroid adenoma with prominent lymphocytic infiltration in a 29-year-old man with hyperparathyroidism for 4 years. Immunohistochemical studies were important in making such a diagnosis.