The effect of age on breast cancer survival is still a matter of controversy. Breast cancer in young women is thought to be more aggressive and to have worse prognosis but results from clinical research have been neither consistent nor definitive. In this study, we have assessed the impact of young age at diagnosis on tumor characteristics, treatment and survival of breast cancer. The study included 82 very young (< or = 35 years), 790 young (36-49), and 2125 older (50-69) women recorded between 1990 and 2001 at the Geneva Cancer Registry. Very young and young patients had more often stage II cancers (P = 0.009), poorly differentiated (P < 0.001) and estrogen receptor negative (P < 0.001) tumors. They were also more likely to receive chemotherapy (P < 0.001) and less likely to receive hormonal therapy (P < 0.001). Specific five-year survival was not different in the three groups (91%, 90%, and 89% for very young, young and older, respectively). When adjusting for all prognostic variables, age was not significantly related to mortality from breast cancer with a hazard ratio of 0.8 (95% CI: 0.3-2.0) for very young and 1.1 (95% CI: 0.8-1.4) for young patients compared to older women. Tumor stage, differentiation, estrogen receptor status, surgery, and radiotherapy were all independent determinants of breast cancer prognosis. We conclude that age is not an independent prognostic factor when accounting for breast tumor characteristics and treatment.