The MEN1 syndrome is a hereditary disease characterized by the simultaneous occurrence of parathyroid, pituitary and duodenopancreatic neuroendocrine tumors. The prevalence of MEN1 is estimated at 3-4/100,000 with a high age-related penetrance of the three main manifestations. The clinical practice guideline for MEN1 recommends a strict screening protocol from an early age. The first part of this thesis elaborates on familial screening and the consequence of novel findings with regards to screening. The second part describes the impact of having the MEN1 syndrome and subsequent screening of the disease and its manifestations. Chapter 2 reveals the morbidity and mortality arising from lag times from diagnosis of the index case and subsequent family members. In general, individuals with longer lag times had more morbidity. It is concluded that an early diagnosis in relatives is imperative and will lead to less morbidity. The whole family should be screened within one year after a germline mutation is identified in the index case. It was previously proposed that blood type O was associated with a higher occurrence of neuroendocrine tumors in MEN1 and that blood type could be a useful addition to the current screening program. Chapter 3 shows that there was no association between blood type O and neuroendocrine tumors in the Dutch MEN1 cohort. Addition of the blood type to the screening program therefore seemed not of additional value. A recent study revealed an association between MEN1 and an early-onset elevated relative breast cancer risk of 2.83. Chapter 4 presents the results of a study assessing whether other risk factors were associated with this higher risk. The analysis revealed that breast cancer occurred at the age of 57.5 years in women without MEN1 and 45 years in women with MEN1 (P=0.03). Surveillance from the age of 40 seemed most appropriate. A biennial surveillance program was deemed justifiable considering the luminal type of breast cancer with a favourable disease course, which holds for the ...