Multiple endocrine neoplasia type 1(MEN 1) is an autosomal dominantly inherited syndrome, characterized by the combined occurrence of tumors of the parathyroid glands, endocrine pancreas, and anterior pituitary gland. The MENIN gene, which is a kind of tumor suppressor gene, is located at the chromosomal locus 11q13. It consists of one untranslated exon and nine exons encoding the menin protein. We report a case of a 22-yearss-old woman with MEN type 1, who was proven to have a mutation in the MENIN gene. The patient was admitted because of repeated hypoglycemia. The fasting plasma glucose level was 32mg/dL. Seventy two hours fasting test showed an the insulin/glucose ratio as 0.33. Endoscopic ultrasonography detected multiple masses on the pancreas. The arterial -stimulated venous sampling(ASVS) with calcium showed sudden step up of insulin at the head and tail portions of the pancreas. The sellar MRI showed a pituitary mass that produced prolactin. Instead of a pathologic diagnosis from operational specimen, the genetic analysis revealed a mutation in the MENIN 1 gene(exon 2, 200~201insAGCCC).