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Various groups have called for a national system to monitor the quality of cancer care. The validity of cancer registry data for quality of cancer care has not been well studied. We investigated the validity of such information in the California Cancer Registry.We compared registry data associated with care with data abstracted from the medical records of patients diagnosed with breast cancer. We also calculated a quality score for each subject by determining the proportion of four evidence-based quality indicators that were met and then compared overall quality scores obtained from registry and medical record data. All statistical tests were two-sided.Records of 304 patients were studied. Compared with the medical record data gold standard, the accuracy of registry data was higher for hospital-based services (sensitivity=95.0% for mastectomy, 94.9% for lumpectomy, and 95.9% for lymph node dissection) than for ambulatory services (sensitivity=9.8% for biopsy, 72.2% for radiation therapy, 55.6% for chemotherapy, and 36.2% for hormone therapy). On average, quality scores calculated from registry data were 11 percentage points (95% confidence interval [CI]=9 to 13 percentage points, P<.001) lower than those calculated from medical record data. Quality scores calculated from registry data were 5 percentage points (95% CI=3 to 7 percentage points) lower for patients with stage I breast cancer, 16 percentage points (95% CI=12 to 20 percentage points) lower for patients with stage II breast cancer, and 20 percentage points (95% CI=8 to 32 percentage points) lower for patients with stage III breast cancer than were corresponding scores calculated from medical record data (all P<.001). The greater difference in quality scores for stage II and III patients revealed that disease severity and setting of care affected the validity of registry data.Cancer registry data for quality measurement may not be valid for all care settings, but registries could provide the infrastructure for collecting data on the quality of cancer care. We urge that funding be increased to augment data collection by cancer registries.