Objective: Identifying a germline mutation in the multiple endocrine neoplasia type 1 (MEN1) gene in an index case has consequences for a whole family. Eligible family members should be offered genetic counseling and MEN1 mutation testing. Subsequently, clinical screening of mutation carriers according to the guidelines should be initiated. We assessed whether there is a lag time from MEN1 diagnosis of the index case to MEN1 diagnosis of family members. In addition, we determined whether this lag time was associated with an increased morbidity and mortality risk. Design: A cohort study was performed using the Dutch MEN1 database, including >90% of the Dutch MEN1 population >16 years of age (n = 393). Results: Fifty-eight MEN1 families were identified, of whom 57 were index cases and 247 were non-index cases (n = 304). The median lag time in MEN1 diagnosis of family members was 3.5 (range, 0-30) years. At the time of MEN1 diagnosis, 30 (12.1%) non-index cases had a duodeno-pancreatic neuroendocrine tumor, of whom 20% had metastases with a mean lag time of 10.9 years, in comparison with 7.1 years without metastases. Twenty-five (10.1%) non-index cases had a pituitary tumor, of whom 80% had a microadenoma and 20% had a macroadenoma, with mean lag times of 7.2 and 10.6 years, respectively. Ninety-five (38.4%) non-index cases had a primary hyperparathyroidism with a mean lag time of 9.5 years in comparison with seven patients without a primary hyperparathyroidism with a mean lag time of 3 years (P = .005). Ten non-index cases died because of a MEN1-related cause that developed during or before the lag time. Conclusion: There is a clinically relevant delay in MEN1 diagnosis in families because of a lag time between the diagnosis of an index case and the rest of the family. More emphasis should be placed on the conduct of proper counseling and genetic testing in all eligible family members.