Objective To assess the impact of substance abuse claims redaction on Medicare spending estimates for beneficiaries with serious mental illness. Data Sources The 2012 claims and unredacted beneficiary‐level Medicare spending totals from CMS's Chronic Conditions Warehouse. Study Design We identified beneficiaries with claims affected by the redaction by comparing claims‐based spending estimates to unredacted spending totals. Differences in characteristics of beneficiaries with and without redacted claims were examined in bivariate analyses. Principal Findings Claims‐based spending totals differed from unredacted totals for 19.7 percent of the cohort. Part A spending for those with redacted claims was underreported by 57.0 percent. Characteristics of beneficiaries with and without redacted claims differed significantly. Conclusions Researchers who rely on Medicare claims to analyze spending outcomes for beneficiaries with serious mental illness should be aware of the potential for bias due to nonrandom redaction of substance abuse data.