The value of individual prognostic factors may change dependent on the length of the follow-up period, if some factors have their greatest prognostic potential immediately after operation. It is not clear how long these factors keep their prognostic relevance. We retrospectively examined data on 1423 surgically treated Japanese patients with primary breast cancer between 1983 and 2002. Survival analysis was done starting at 2.5-yearly intervals after operation and follow-up in the first analysis started at the time of the operation. The changing importance of the prognostic factors during different follow-up periods was investigated by univariate and multivariate analysis. Based on multivariate analysis, tumor size retained its prognostic value even up to 7.5 years after operation, whereas the age, vascular involvement, ER and PgR showed a changing influence on prognosis dependent on the length of the follow-up period. The prognosis of patients some years after operation is necessarily different from the initial prognosis established after operation. Detecting the changing importance of prognostic factors could provide new biological insights that might otherwise be missed, and may help determine the most appropriate clinical use of various factors.