Between April 1990 and December 1997, 811 consecutive patients with 830 newly diagnosed breast cancers having their primary treatments in our institution were included in this study. Sixty three percent of breast cancer patients were premenopausal. The early-onset breast cancer (age < or = 40) composed 29.3% of all patients. The five-year survival rate of all patients was 80.4% (95% confidence interval [CI], 76.2-84.6%). The five-year overall survival rate for stage 0 was 95.7% (95% CI, 87.3-100%), stage I, 93.9% (95% CI, 88.9-98.9%), stage II, 88.5% (95% CI, 82.0-95.1%), stage III, 65.0% (95% CI, 54.0-75.9%), and stage IV, 18.5% (95% CI, 3.4-33.7%). Multivariate analysis of primary operable breast cancer revealed that axillary lymph node involvement, high nuclear grade and early-onset breast cancer (age < or = 40) were poor prognostic factors. The early-onset breast cancer had a more aggressive clinical behavior than that of the older age group, their five-year disease-free survival rates for stage I, stage II and stage III diseases being only 64.7%, 66.5%, and 43.3%, respectively. In these patients the only meaningful prognostic factor was extensive axillary lymph node metastasis (> or = 10). In summary, breast cancer patients in Taiwan tend to be younger than their counterpart in western countries. The early-onset breast cancer had poorer prognostic features for all stages comparing to the older age group. Standard pathologic factors are not good predictors of their outcome. For these patients new biologic markers need to be sought to distinguish between high and low risk and the treatment strategy for them should be guided by the aggressive characteristics of the disease.