We describe a patient with multiple endocrine neoplasia type 1 characterized by the simultaneous occurrence of parathyroid cancer, parathyroid adenomas, and pancreatic gastrinoma, who presented with an episode of acute hypercalcemia. The rapid parathyroid hormone assay provided a basis for the diagnosis of parathyroid hyperfunction. Mediastinal metastasis of the parathyroid carcinoma was found at autopsy. However, the staining of pancreatic and gastric tissue for parathyroid hormone-related protein does not make it possible to exclude completely the contribution of this peptide in mediating the hypercalcemia. To our knowledge, this is the first reported case of parathyroid carcinoma as part of the multiple endocrine neoplasia type 1 syndrome.