Context: The natural course and survival of neuroendocrine tumors (NETs) of thymus (Th) and lung in multiple endocrine neoplasia type 1 (MEN1) patients are still unknown. Objective: Our objective was to assess prevalence, tumor growth, and survival of Th and lung NETs in an unselected MEN1 population with long-term follow-up. Design: This was an observational study. Patients and Methods: A longitudinal study was performed using the Dutch national MEN1 database, including >90% of the Dutch MEN1 population >16 years of age. Patients under care of the Dutch University Medical Centers (1990-2011) (n = 323) were included. Main Outcome Measures: The prevalence and survival of Th and lung NETs were assessed. Linear mixed-models analysis was applied to assess tumor growth with age as a possible confounder and gender, genotype and baseline tumor size as possible effect modifiers. Results: Th NETs occurred in 3.4% of patients, almost exclusively in males with a 10-year survival of 25% (95% confidence interval = 8%-80%). Athoracic computed tomographys can was available in 188 patients (58.2%). A lung NET was identified in 42 patients (13.0%) with a 10-year survival of 71.1% (95% confidence interval = 51%-100%). Tumor volume of lung NETs increased 17% per year (P < .001) (tumor doubling time 4.5 years). Tumor doubling time in males was 2.5 vs 5.5 years in females (P = .05). Lung NET growth was not associated with genotype or with baseline tumor size (< 1 vs >= 1 cm). Conclusion: In MEN1 patients, Th NETs almost exclusively occurred in males and had a very low prevalence and a high mortality. Lung NETs occurred more often than previously thought, had an indolent course, and occurred equally in both sexes. Tumor growth in males was double compared with female patients.