Germ-line mutations of the MEN1 gene were analyzed in five cases of familial and four cases of sporadic multiple endocrine neoplasia type 1 (MEN-1), six cases in three independent pedigrees of familial pituitary adenoma without MEN-1, and three cases of familial isolated primary hyperparathyroidism (FIHP) in Japanese. Eight different types of germ-line mutations in all nine cases of MEN-1 were distributed in exons 2, 3, 7, and 10 and intron 7 of the MEN1 gene. Loss of heterozygosity (LOH) on 11q13 was detected in all nine tumors of these cases with microsatellite analysis. No germ-line mutation of the MEN1 gene was detected in three pedigrees of familial pituitary adenoma and three cases of FIHP. LOH on 11q13 was detected in two cases in one pedigree of familial pituitary adenoma, and one of them showed a heterozygous somatic mutation of the MEN1 gene. No LOH on 11q13 was detected in three cases of FIHP. Based on these, we conclude that the loss of function of menin is etiological for familial or sporadic MEN-1, but not for FIHP or most familial pituitary adenoma without MEN-1.