Multiple endocrine neoplasia type 1 (MEN 1) is an autosomal dominant inherited disorder characterized by tumors of the enteropancreas, parathyroid and anterior pituitary. The MEN 1 gene was recently cloned, and germline mutations of the gene have been demonstrated in cases of MEN 1. Here, we report a Japanese family with a germline mutation of the MEN 1 gene. The proband (44 y.o., male) had primary hyperparathyroidism (PHP) and pancreatic carcinoid, and his older sister (50 y.o.) had a history of parathyroidectomy for primary hyperparathyroidism at the age of 40. Clinical examination revealed no evidence of PHP or other MEN 1-related tumors in his son. Direct sequencing analysis revealed a heterozygous germline mutation (1001delC) at codon 297 in exon 6 of the MEN 1 gene in the proband and his son. Loss of heterozygosity (LOH) was also found in the resected parathyroid tissue of the proband. The deletion of cytosine 1001 observed in this case induces a frame shift, which causes the appearance of a stop codon (TAG) at codon 367. This mutation appears to be associated with tumors of the endocrine tissues in the cases studied.