PURPOSE: American Joint Committee on Cancer (AJCC) Tumor (T), Nodal (N), and Metastatic (M) staging is commonly used in clinical practice for treatment decisions, yet before 2004, Surveillance Epidemiology and End Results (SEER)-affiliated cancer registries did not routinely include TNM staging defined by AJCC criteria, reporting instead SEER Summary Staging. METHODS: We developed and validated an algorithm to determine AJCC TNM staging from Extent of Disease information for 17,133 female breast cancer cases diagnosed from 1988 to 2003 in the cancer registries of Kaiser Permanente Northern and Southern California. Test characteristics (percent agreement, Cohen’s kappa, sensitivity, specificity) were calculated to compare derived TNM with gold-standard TNM available in the registry. RESULTS: Agreement for TNM variables was excellent (range 0.91-1.00 for percent agreement and Cohen’s kappa). The sensitivity and specificity, respectively, of the algorithm for AJCC TNM Version 6 staging were as follows: Stage 0 (0.99, 1.00), Stage I (0.97, 0.98), Stage II (0.91, 0.96), Stage III (0.69, 0.99), and Stage IV (0.92, 1.00). Stage III had lower sensitivity due to reclassification of supraclavicular lymph node positivity from M1 (Stage IV) to N3 (Stage IIIC) in AJCC Version 6. CONCLUSIONS: Derived AJCC staging for breast tumors diagnosed before 2004 is feasible and accurate using cancer registry data.