BACKGROUND: Multiple endocrine neoplasia type 1 (MEN 1) is a syndrome with tumors of many endocrine tissues. Germline MEN1 gene mutations were found in most patients with familial or sporadic MEN 1. Recently, somatic MEN1 gene mutations were also detected in sporadic non-MEN 1 endocrine tumors. METHODS: We used direct sequence analysis to investigate MEN1 gene mutations in 30 parathyroid tumors obtained from 30 patients with sporadic, nonfamilial primary hyperparathyroidism. RESULTS: Four patients had somatic mutations of the MEN1 gene, comprising 1 small insertion (1091insAGC), one missense mutation (G42S), and 2 non-sense mutations (E388X, R460X). Identical missense and non-sense mutations were found in patients with familial and non-familial MEN 1. There were no differences between clinical features of patients with and without MEN1 gene mutations. CONCLUSIONS: The incidence of somatic MEN1 gene mutations (13.3%) in Japanese patients with sporadic, nonfamilial primary hyperparathyroidism is almost equal to those of such patients in the United States and Sweden. Occasionally, the MEN1 gene mutation sites in sporadic parathyroid tumors are identical to those reported in tumors from patients with familial or sporadic MEN 1.