16 Background: The cost of cancer care rose dramatically over the past decades, increasing the financial burden on patients. However, current estimates of healthcare spending and out-of-pocket burden rely heavily on data from older populations that may not reflect contemporary treatment patterns in working-aged adults or variation in type of insurance coverage. Methods: We used the 2012-2021 Colorado Central Cancer Registry linked to the All-Payer Claims Database to identify adults aged 22-63 years newly diagnosed with invasive cancer who were consecutively enrolled on the same insurance type or the month of diagnosis and 6 months after (or until death). We used quantile regression to adjust spending by age and sex and report median insurer reimbursements for medical care and pharmacy costs and out-of-pocket spending (copays, deductibles, coinsurance) for the most common insurance and cancer types. Results: Among 31,179 individuals newly diagnosed with cancer, the median total spending six months after diagnosis across all cancers ranged from 59,022 for those insured by private non-health maintenance organization (HMO) plans to 26,887 for those enrolled on Medicaid and 19,152 for those on Medicare Advantage (MA) plans (Table). The majority of spending was for medical care. Among individuals insured by private non-HMO plans, those with lymphoma had the highest total median spending at 130,408, followed by lung (94,207), breast ($87,830), and leukemia ($82,445). Although total spending was markedly lower among those insured by Medicaid, among this group, individuals with leukemia ($50,399) and lymphoma (48,747) experienced the highest median spending. Out-of-pocket spending was substantially higher for those insured by private non-HMO plans (3,582 or approximately 511/month) across all cancer types, and highest for lymphomas (3,966) and lung ($3,455) cancers. Conclusions: Total healthcare spending in the 6 months after a cancer diagnosis is considerable in the working-age population, with a high out-of-pocket burden for patients with private non-HMO coverage. Median spending in 6 months after cancer diagnosis (in 2021 dollars), adjusted for age and sex. Medical Care Plan Payments* Pharmacy Plan Payments* Out of Pocket Spending* Total Spending* Overall(n=31,793) Dual Medicaid/Medicare $24,081 $1,793 $19 $28,806 Medicaid $23,415 $641 $13 $26,887 Medicare TM $20,698 $797 $3,275 $27,028 Medicare MA $14,227 $570 $2,069 $19,152 Private non-HMO $51,196 $274 $3,582 $59,022 Private HMO $36,414 $219 $2,622 $42,916 Median costs significantly different across all payers, p<0.05.