BACKGROUND: Numerous studies have used the SEER or Medicare data to assess the use of radiation therapy. However, the completeness of these data has not been evaluated. METHODS: Using linked SEER-Medicare data, a cohort was created that included incident cases of breast, endometrial, lung, prostate, or rectal cancer from 1991 to 1996 in persons 65 years and older. For each of these cases, use of radiation therapy as an initial course of treatment reported from the SEER data was compared with use of radiation therapy in the months following diagnosis, as determined from the Medicare claims. RESULTS: Overall, there was a high agreement between SEER and Medicare sources for use of radiation therapy. Agreement was 88% for lung cancer, 93% for prostate cancer, 94% for rectal and breast cancers, and 95% for endometrial cancer. Within the Medicare files, radiation claims were found in MedPAR Physician Supplier Part B and Outpatient files. CONCLUSIONS: There is a high level of agreement between SEER and Medicare reporting of radiation treatments after a cancer diagnosis, suggesting that either source can be used to assess radiation-related treatment patterns. However, for the most complete ascertainment of radiation use, both SEER and Medicare sources should be used.