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Background: Systemic mastocytosis (SM), a rare condition affecting about 32,000 individuals in the United States, is often misdiagnosed or underdiagnosed owing to its nonspecific symptoms and the need for invasive biopsies. Objective: Our aim was to identify, classify, and characterize the natural history of patients with SM. Methods: In a retrospective cohort study, administrative data from a large managed care organization was used to identify patients with confirmed SM, based on World Health Organization criteria. Demographic data, delay to diagnosis, disease progression, and health care resource utilization were examined. Results: Of 116 patients with confirmed SM, 77% had indolent SM, 2% had smoldering SM, 12% had SM with associated hematologic neoplasm, 9% had aggressive SM, and none had mast cell leukemia. In all, 5 patients were misclassified as having a less advanced SM subtype initially and 3 were completely undiagnosed (missed diagnosis). The average delay to diagnosis of SM was 58.3 plus or minus 73.1 months. In all, 18% of patients progressed from a nonadvanced form of SM (indolent or smoldering SM) to an advanced form of SM (aggressive SM, SM with associated hematologic neoplasm, or mast cell leukemia) over an average of 88.3 plus or minus 82.7 months. Patients with SM had increased health care utilization, including increases in their numbers of hospital admissions, emergency room visits, urgent care visits, and specialty provider visits, after diagnosis versus before. Conclusions: Rare diseases such as SM would benefit from increased understanding and awareness to improve diagnostic accuracy. Prospective studies are needed to better characterize this patient population and determine the type of follow-up needed to recognize advanced forms of SM so that appropriate treatment can be implemented.