Background: Most short-term limited duration (STLD) insurance plans do not cover essential health services, such as cancer screenings, which may delay cancer diagnoses. STLD plans can disrupt individual insurance markets, leading to higher costs and delays in care for comprehensive plan enrollees. In October 2018, federal regulations extended the allowable duration of STLD plans to up to 36 months. States have taken varying approaches to expand, restrict, or eliminate the sale or terms of STLD plans, both prior to 2018 and following its enactment. Our study examined associations of state-level policies regarding STLD plans and late-stage cancer diagnosis before and after the 2018 change. Methods: We identified 1,289,366 adults aged 18-64 years newly diagnosed with cancer in 01/2016-02/2020 in 47 states and DC from the National Cancer Database, with a “washout” period +/- 3 months of the 2018 regulation. Patients were categorized into 4 groups according to their states’ STLD plans policies: (1) states prohibiting STLD plans prior to 2016; (2) states banning STLD plans or strict regulations leading insurers to stop offering STLD plans after 2018; (3) states restricting STLD plans; (4) states with no STLD regulation. We conducted difference-in-differences (DiD) analyses to evaluate changes in late-stage diagnoses (stages III/IV) pre- (01/2016-06/2018) and post- (01/2019-02/2020) STLD plan expansion between groups 2, 3, 4 vs group 1 for all cancers combined and 2 common cancers with effective screening tests: female breast and colorectal. We adjusted for age group, sex, race/ethnicity, rurality, area-level poverty, and state random effects to account for other state-level variations. Results: Percentages of late-stage diagnosis decreased since 2018 in all state groups (Table). Compared to patients in group 1 states (prohibited STLD plans prior to 2016), group 4 (no STLD regulations) had a net increase of 0.8 (95%CI = 0.2-1.3) percentage points (ppt) in late-stage diagnosis and group 3 (some STLD regulations) had a net increase of 0.9 (95%CI = 0.3-1.4) ppt in late-stage diagnoses (Table). Similar patterns were observed for female breast and colorectal cancers. Conclusions: The federal policy on STLD plan expansion was associated with an increase in late-stage cancer diagnoses, underscoring the importance of individual state policies banning or restricting STLD plans.