We report three sporadic parathyroid tumors with biallelic inactivation of the multiple endocrine neoplasia type 1 (MEN1) gene. Three parathyroid tumors had two somatic mutations (K119del and 864del8, 363insT and 1767delT, and 508del33 and W341X, respectively). The mutations in both alleles detected by long-range polymerase chain reaction and subcloning in three tumors would likely result in a nonfunctional menin protein in parathyroid glands. These results show that the MEN1 gene is inactivated not only by a combination of somatic mutations and loss of heterozygosity, but also by somatic double mutations located on different alleles. The results directly confirmed the participation of MEN1 in the tumorigenesis of sporadic parathyroid tumors.