Despite numerous studies, the effect of patient age on the prognosis of breast cancer is still uncertain. The aim of this study was to assess the influence of age on long-term relative survival, to control the results for the extent of disease at diagnosis and assess the association between biological markers and age of the patients. A population-based survival study was made to assess the 5- and 10-year relative survival. All 17,856 female breast cancer patients diagnosed in Finland and reported to the Finnish Cancer Registry in 1977-1986 were included. The results were controlled for the extent of the disease. The markers of biological aggressiveness of tumours and patients' age were correlated in a prospectively collected subset of 2107 patients from the Tampere University area. The relative 5-year and 10-year survival rates (RSRs) were highest in women 46-50 years of age, whereas there was no significant difference between younger and older age groups. No consistent survival trends were observed among the age groups in local, node-negative disease, whereas in node-positive disease the 10-year relative survival was best for women 41-45 years (49%) and poorest in women over 75 years (35%). The youngest age groups were significantly more often oestrogen receptor-negative, but only small differences were observed for S-phase fraction and progesterone receptor positivity.