PURPOSE: Nearly half of all breast cancers are treated with breast conserving therapy (BCT). The purpose of this study was to identify the risk factors for ipsilateral breast tumor recurrence (IBTR) after BCT in T1 and T2 breast cancer patients. METHODS: The medical records of 294 T1 or T2 breast cancer patients who underwent BCT at Seoul National University Hospital between January 1998 and December 2002 were retrospectively reviewed. Kaplan-Meier curves and Cox proportional regression analysis were used to identify the significant clinicopathologic factors that influence IBTR. RESULTS: Among the 294 patients, 12 patients (4.8%) developed IBTR after a median follow-up of 82 months. Univariate analysis demonstrated that younger age (< or =35 year) had significant associations with IBTR (p=0.006). Tumor size, lymph node status, histologic grade, extensive intraductal component, lymphovascular invasion, and close resection margins were not significant factor associated with IBTR. The triple negative breast cancer subtype also did not have significant association with IBTR. Multivariate analysis showed that the younger age at diagnosis was a significant predictor of IBTR with a HR of 3.86 (p=0.036; 95% CI, 1.09-13.60). CONCLUSION: Younger age at diagnosis (< or =35) may be associated with an increased risk of IBTR in patients who underwent BCT.