Data regarding young age as an independent prognostic factor have been conflicting. We investigated this variable in 696 premenopausal Vietnamese and Chinese women with operable breast cancer who participated in a clinical trial of adjuvant surgical oophorectomy and tamoxifen. Tumor size and axillary lymph node status did not vary with age. Women < 35 years had a greater fraction of histologic grade III tumors (P = 0.06), and in the two thirds of patients with available data, in women < 35 years, there was a lower percentage of estrogen- and progesterone receptor-positive tumors and a higher percentage of HER2/neu-positive tumors (P > 0.14 for each group). In univariate analyses, compared to women > or = 45 years, women < 35 years and 35-39 years were at greater risk for death (P = 0.002 and P = 0.023, respectively), and compared to women > or = 40, women < 40 were at greater risk of death (P = 0.002). Multivariate analyses supported a conclusion that younger age was an independent adverse prognostic factor for survival (P = 0.005, age as a continuous variable). Kaplan-Meier analyses in all patients and in oophorectomy and tamoxifen-treated patients, but not in observation-only patients, showed statistically significant poorer disease-free and overall survival in women < 40 years compared to those > or = 40 years. Thus, despite efficacy of the combined adjuvant hormonal therapy, younger age was a risk factor for poorer survival.