Previous studies have reported significantly worse outcomes in younger women with breast cancer. The cut-off for young age differed between studies and it is unclear whether the survival-age relationship is a linear function across age in premenopausal women. We analyzed data on 9,885 breast cancer patients aged <or=50 years who entered the Korean Breast Cancer Society Registration Program between 1992 and 2001. Overall survival data were compared among age groups. The median follow-up duration was 74.4 months. Of all patients, 351 were aged <30 years (Group I), 1,092 were aged 30-34 years (Group II), 2,087 were aged 35-39 years (Group III), and 6,354 were aged 40-50 years (Group IV). In univariate and multivariate analyses, Group I patients showed a worse outcome than Group II patients, and Group II patients showed a worse outcome than Group III patients. Interestingly, the survival rates of Group III and Group IV patients did not differ significantly. This survival trend according to age group was not found in the subgroup of patients with hormone receptor-negative tumors. In patients aged <35 years, the risk of death rose by 5% for every 1-year reduction in age, whereas there was no significant change in death risk with age in patients aged 35-50 years. We show that risk of death from breast cancer increases sharply in women younger than 35 years. This suggests that age less than 35 years is a reasonable cut-off for defining young age-onset breast cancer.