Multiple endocrine neoplasia syndrome, type 1 (MEN1) is an underdiagnosed autosomal dominant inherited cancer predisposition syndrome with inter- and intrafamilial variability without a known genotype-phenotype correlation. Disease is caused by mutations in the MEN1 gene located on chromosome 11, but other genes (CDKN1B, AIP) and mechanisms might be involved too. We performed retrospective case series study of MEN1 syndrome patient and his family and present our experience of management of genetically confirmed 9 MEN1 syndrome large family members from Lithuania with novel MEN1 gene mutation (MEN1 exon 6 - c. 879delT (p.Pro293Profs*76)) and delineate its clinical phenotype. At present the diagnosis of MEN1 syndrome must be established by direct mutation testing. MEN1 syndrome patients, their relatives and patients suspected of MEN1 are eligible for mutation testing. Patients with MEN1 have a shorter life expectancy than the general population. MEN1 patients and mutation carriers should be subjected to periodic screening in order to detect manifestations in an early stage. Early genetic diagnosis and subsequent periodic screening is associated with less morbidity and mortality at follow-up. Our study confirmed the absence of genotype-phenotype correlation and showed high intrafamilial clinical expression variability of the MEN1 syndrome.