Young women with breast cancer have a more unfavorable outcome and advanced disease than older women. This study was initiated to determine the difference in tumor biology between younger and older groups. One hundred fifty-five patients with invasive ductal carcinoma, not otherwise specified, comprised the study group, including 50 women aged less than 35 years, 50 aged 36 to 50 years, and 55 aged more than 50 years. Histopathologic parameters, including tumor size, combined histologic grade, and axillary lymph node status, were studied. Biomarkers, including estrogen receptor status, tumor proliferation rate as determined by Ki-67, and gene expressions of c-erbB-2, p53, bcl-2, and BRCA1, were determined by immunohistochemistry. Comparisons of the distribution of these parameters in three age groups were performed. Breast cancer occurring in women aged less than 35 years had a significantly higher incidence of large tumor, high proliferation rate, and loss of nuclear BRCA1 expression (44.0% versus 22.0% or 23.6%) than in the two older age groups. Breast cancer in women aged less than 35 years also had higher histologic grade and higher frequency of bcl-2-negative tumor than that found in the 36- to 50-year age group. No difference was found in lymph node status and c-erbB-2 and p53 gene expressions between the age groups. Loss of BRCA1 nuclear expression significantly correlated with higher histologic grade and high Ki-67 index (P < 0.05) in group A. These findings suggested that women aged less than 35 years have frequent loss of nuclear BRCA1 expression, which may be responsible for the specific tumor biology different from older women. However, c-erbB-2 and p53 gene expressions seem to have no important role in the adverse tumor behavior of breast cancer in young women.