In this paper 25-year survival for 8802 Norwegian women with breast cancer diagnosed during the period 1965 to 1974 is studied. It is suggested that some of the contradictory reports in the literature of the prognostic effect of age and clinical stage on long-term survival may be caused by interactions and time varying effects of covariates. When using a linear non-parametric regression model that allows the covariates to vary over time, age and clinical stage are found to be significant long-term prognostic factors. A significantly higher excess mortality for women less than 35 years at diagnosis disappeared after 8 years, while for those above 55 years an important effect of age on the long-term survival, especially for those with regional cancer, was seen. The effect of clinical stage on survival varies strongly over time, and was significant between 15 and 20 years.