PURPOSE: Since the publication of the 5th edition of the AJCC cancer staging manual in 1997 (old stage), significant developments have occurred in the field of breast cancer diagnosis and management; therefore, it was revised at 2002 (new stage). There are few reports comparing the changes in prognosis in relation to the changes in the staging system. The aims of this study were to evaluate the changes in patient distribution and prognosis according to the changes in the staging system and to elucidate the efficacy of new staging system. METHODS: The records of 1, 275 patients who underwent an operation for breast cancer at Yeung-Nam University Hospital between 1987 and 2003 were reviewed. The pathological stage was assigned retrospectively according to the 5th and the 6th AJCC staging criteria. The patient distributions by stage, nodal status, 5-year relapse free survival (RFSR) and overall survival rates (OSR) were retrospectively compared. RESULTS: Five hundred and five of 616 stage II patients according to the 1997 classification system were also stage II according to the 6th AJCC staging system. The number of patients with stages IIA and IIB decreased from 370 and 246 (old stage) to 342 and 165 (new stage), respectively. Conversely, the number of patients with stage III increased from 158 (old stage) to 271 (new stage). The five-year RFSR for patients with stage I, IIA, IIB, and IIIA were 94.2, 87.1, 74.3, and 48.8% according to the old stage (P<0.0001), and 95.2, 87.8, 81.7, and 66.8%, respectively, according to the new stage (P<0.0001). The five-year OSR for patients with stage I, IIA, IIB, and IIIA were 98.7, 94.3, 86.1, and 63.5% according to the old stage (P<0.0001), and 98.7, 95.7, 96.5, and 72.9%, respectively, according to the new stage (P<0.0001). The RFSR and OSR for stage IIIC were 42.0 and 59.5%, respectively. There was significant difference in the five-year OSR for stages IIB and IIIA (P=0.0308 and P=0.0132, respectively). CONCLUSION: In our study, the 6th AJCC staging system shifted poorer prognostic cohort of each stage toward a higher stage compared to the 1997 version. Therefore, the survival rate for any one stage assigned by 2002 staging system was also improved. In conclusion, it is imperative that careful attention is devoted to this effect so that accurate conclusions regarding the efficacy of new treatment can be drawn.