Background: Healthcare accrediting organizations and insurers increasingly require reporting of clinical data, and cancer treatment is one area of enhanced scrutiny. Objectives: To compare rates of received versus reported adjuvant breast cancer treatments, and to assess barriers to measuring and reporting treatments to the tumor registry (TR) of a high-volume medical center with both hospital-based and community-based oncologists. Research Design: We calculated rates of received treatments using data collected using chart abstraction (N = 115) and compared these with rates of reported treatments from the TR (N = 535). We conducted 31 indepth interviews with clinical and administrative informants. Asking about perceptions of the TR, current reporting methods, and reporting barriers. Interviews were recorded, transcribed, and analyzed using deductive and inductive methods. Results: Rates of reported versus received treatments were radiation therapy after breast-conserving surgery 22% versus 84% (P < 0.0001); chemotherapy for stage 2 or 3: 17% versus 79% (P < 0.0001); hormonal therapy for stage 2 or 3: 1% versus 91% (P < 0.0001). Comparing community-based versus hospital-based oncologists' rates reported to the TR, we found the following differences: radiation therapy post-breast conserving surgery 12% versus 32% (< 0.0001); chemotherapy 8% versus 29% (< 0.0001); and hormonal therapy 0% versus 3% (0.09). We found 4 key barriers to measuring and reporting poor understanding about the TR, limited information technology capabilities, poor communication, and mistrust. Conclusions: Efforts to improve cancer care quality by improved treatment reporting must overcome key barriers, especially those involving information exchange and mistrust. Communications between the TR and oncology practices must improve to facilitate better treatment measurement and reporting.