Neurofibromatosis type 1 (NF1) is a dominantly inherited multiorgan genetic condition caused by pathogenic variants of the NF1 gene. NF1 is characterized by café au lait macules, axillary and inguinal freckling, and cutaneous neurofibromas. The syndrome also predisposes affected persons to both benign and malignant tumors. Cognitive and behavioral problems are also common in NF1. The birth incidence of NF1 is approximately 1/2,000, and the mean prevalence is 1/4,000 owing to compromised survival. In this thesis, a Finnish nationwide cohort of NF1 individuals was used in three population-based analyses. The data were linked with multiple national administrative health registers to study the distribution of gastrointestinal tumors, gastrointestinal stromal tumors (GISTs) as a cause of death, essential and secondary hypertension, and the risk and prognosis of myocardial infarction in individuals with NF1. The site-specific register analysis of 1,410 NF1 patients revealed a markedly high hazard ratio (HR) of 15.6 for tumors of the small intestine, and the analysis of death certificates demonstrated substantial morbidity related to GISTs in NF1. Cardiovascular outcomes showed that NF1 patients had significantly elevated hazard for both secondary (HR 3.76) and essential hypertension (HR 1.73), with essential hypertension being predominant and occurring an average of six years earlier compared to matched controls. NF1 patients also had an increased risk of myocardial infarction and their survival after myocardial infarction was worse than in controls. The results highlight the importance of early and accurate diagnosis of NF1-related comorbidities, surveillance, multidisciplinary care, and risk reduction to improve patient outcomes. The study has clinical relevance and it may help in formulating the national guidelines of treatment and follow-up of NF1 patients.