The MENI gene has recently been cloned as the gene responsible for multiple endocrine neoplasia type 1 (MEN1) and its germline mutations have been identified in a number of familial MEN1 patients. However, mutation-negative cases have also been reported in some MEN1 families. We report here a Japanese MEN1 family, including a proband with no evidence of MEN1 gene mutation. The proband (51 y.o., female) had three major MENI lesions, including primary hyperparathyroidism (HP), prolactinoma, and pancreatic tumor. Her father and brother had HP, and her daughter had both HP and prolactinoma. When we analyzed the proband for a germline mutation of the MEN1 gene, the direct sequencing analysis showed no mutation in the coding region, on the promoter, 5' and 3'' untranslated regions of the MEN1 gene. We next examined the loss of heterozygosity (LOH) in the proband's parathyroid tumors using two benign polymorphisms (C2249G in intron 1 and 2248del3 in exon 10) in the MEN1 gene to detect LOH. LOH was not found in any of the four separate regions of the tumor tissues.