BACKGROUND: Medicare is the single largest purchaser of laboratory testing in the United States, yet test results associated with Medicare laboratory claims have historically not been available. OBJECTIVE: The purpose of this study was to describe both the linkage of laboratory results data to Medicare claims and the completeness of these results data. In a subgroup of beneficiaries initiating angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers, we also demonstrate the generalizability of Medicare beneficiaries with laboratory values compared with those without laboratory values. We end with a discussion of the limitations and potential uses of these linked data. METHODS: We obtained information about laboratory orders and results for all Medicare fee-for-service beneficiaries in 10 eastern states who had outpatient laboratory tests conducted by a large national laboratory services vendor in 2011. Using a combination of direct identifiers and patient demographic characteristics, we linked patients in these laboratory data to Medicare beneficiaries, enabling us to associate test results with existing claims. RESULTS: Nearly all patients in the laboratory data were able to be linked to Medicare beneficiaries. There were over 2 million distinct beneficiaries with nearly 125 million specific test results in the laboratory data. For specific tests ordered in an outpatient or office setting in these 10 states, between 5% and 15% of them had linked laboratory data. Medicare beneficiaries initiating angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers who had laboratory results data had similar patient characteristics to those without results data. CONCLUSIONS: This novel linkage of laboratory results data to Medicare claims creates unprecedented opportunities for conducting comparative effectiveness research related to patient safety and quality.