ImportanceOut-of-pocket costs (OOPCs) have been largely eliminated for screening mammography. However, patients still face OOPCs when undergoing subsequent diagnostic tests after the initial screening, which represents a potential barrier to those who require follow-up testing after initial testing.ObjectiveTo examine the association between the degree of patient cost-sharing and the use of diagnostic breast cancer imaging after undergoing a screening mammogram.Design, Setting, and ParticipantsThis retrospective cohort study used medical claims from Optum’s deidentified Clinformatics Data Mart Database, a commercial claims database derived from a database of administrative health claims for members of large commercial and Medicare Advantage health plans. The large commercially insured cohort included female patients aged 40 years or older with no prior history of breast cancer undergoing a screening mammogram examination. Data were collected from January 1, 2015, to December 31, 2017, and analysis was conducted from January 2021 to September 2022.ExposuresA k-means clustering machine learning algorithm was used to classify patient insurance plans by dominant cost-sharing mechanism. Plan types were then ranked by OOPCs.Main Outcomes and MeasuresA multivariable 2-part hurdle regression model was used to examine the association between patient OOPCs and the number and type of diagnostic breast services undergone by patients observed to undergo subsequent testing.ResultsIn our sample, 230 845 women (220 023 [95.3%] aged 40 to 64 years; 16 810 [7.3%] Black, 16 398 [7.1%] Hispanic, and 164 702 [71.3%] White) underwent a screening mammogram in 2016. These patients were covered by 22 828 distinct insurance plans associated with 6 025 741 enrollees and 44 911 473 distinct medical claims. Plans dominated by coinsurance were found to have the lowest mean (SD) OOPCs ($945 [$1456]), followed by balanced plans ($1017 [$1386]), plans dominated by copays ($1020 [$1408]), and plans dominated by deductibles ($1186 [$1522]). Women underwent significantly fewer subsequent breast imaging procedures in dominantly copay (24 [95% CI, 11-37] procedures per 1000 women) and dominantly deductible (16 [95% CI, 5-28] procedures per 1000 women) plans compared with coinsurance plans. Patients from all plan types underwent fewer breast magnetic resonance imaging (MRI) scans than patients in the lowest OOPC plan (balanced, 5 [95% CI, 2-12] MRIs per 1000 women; copay, 6 [95% CI, 3-6] MRI per 100 women; deductible, 6 [95% CI, 3-9] MRIs per 1000 women.Conclusions and RelevanceDespite policies designed to remove financial barriers to access for breast cancer screening, significant financial barriers remain for women at risk of breast cancer.