BACKGROUND: Approximately 5%-10% of patients with diabetes are diagnosed with type 1 diabetes mellitus (T1DM), the incidence and prevalence of which is projected to increase through 2050. Despite this, T1DM-related health care resource utilization (HCRU) and economic burden in the United States have not been adequately assessed, since previous studies used various cost definitions and underlying methods to examine these outcomes. OBJECTIVE: To assess HCRU and costs incurred by patients with T1DM in the United States. METHODS: This retrospective cohort study used IBM Watson MarketScan data from 2011 to 2015 and Optum's electronic medical record (EMR) and integrated data (i.e., linked EMR and administrative claims data) from 2011 to 2016. Included patients had ≥ 1 T1DM diagnosis (the earliest diagnosis date was designated as the index date), were continuously enrolled for ≥ 6 months during their pre-index baseline periods, and had ≥ 1 pharmacy claim for insulin or an insulin pump within ± 90 days of the index date. Baseline demographic and clinical characteristics were summarized descriptively. Average monthly HCRU and costs per patient per month (PPPM) paid by the health plan and patient were assessed. Costs were adjusted for inflation to 2018 U.S. dollars. RESULTS: We identified 181,423 patients with T1DM who met the selection criteria in MarketScan, 84,759 in the Optum EMR, and 8,948 in the Optum integrated databases. Most patients were male (range across databases: 52.6%-53.1%), relatively young (medians: 33-35 years, overall range: 0-100 years), and had a Charlson Comorbidity Index score of 1 (69.2%-73.0%) across all databases. Total all-cause and diabetes-related costs ranged from $1,482 to $1,522 and $733 to $780 PPPM, respectively, during the follow-up period. Pharmacy costs contributed most to the total cost of care, accounting for 55.3% ($431) to 61.1% ($448) of total diabetes-related costs. On an annualized basis, patients had an average of 0.2-0.9 all-cause hospitalizations and 0.1-0.3 diabetes-related hospitalizations during follow-up. The median costs per diabetes-related hospitalization ranged from $6,548 to $8,439, accounting for 4%-7% of total monthly diabetes-related costs. Patients had an average of 0.4-0.5 all-cause and 0.1-0.2 diabetes-related emergency department (ED) visits annually; the median costs of ED visits were $972-$1,499, contributing about 2% of monthly diabetes-related costs during follow-up. CONCLUSIONS: In this large, retrospective, observational study of pediatric and adult patients with T1DM, diabetes-related costs totaled nearly $800 per month. Pharmacy costs contributed to over half of diabetes-related costs, indicating the substantial economic burden associated with the treatment of T1DM. Additional research is needed to determine risk factors associated with costly events (e.g., hospitalizations and ED visits) and indirect costs associated with T1DM. DISCLOSURES: JDRF International provided funding for this project and manuscript. JDRF International also contracted with Evidera, a research and consulting firm for the biopharma industry, for its participation in the project and in the development of this manuscript. The Leona M. and Harry B. Helmsley Charitable Trust provided JDRF International with funding. Simeone, Shah, and Ganz are employed by Evidera and do not receive any payment or honoraria directly from Evidera's clients. LeGrand is an employee of JDRF International. Bushman was employed by JDRF International during the conduct of the study and development of this manuscript. Sullivan and Koralova are employees of The Leona M. and Harry B. Helmsley Charitable Trust.