The aim of this study was to assess the prognostic influence of the month of treatment, the year of treatment, and the patient's age at diagnosis of breast cancer in comparison with clinical and histopathological prognostic factors. This retrospective analysis from the years 1968-1990 at one university hospital in a rural area in Eastern Finland included 688 patients with invasive breast cancer followed up for more than 12 years. The breast tumours diagnosed in September and in October were larger, had distant metastases more often and, accordingly, had a worse prognosis than the tumours diagnosed during the rest of the year. The prognosis of breast tumours improved during the study period whereas the patient's age was inversely related to prognosis. The histopathological characteristics of breast tumours were not related to the month of treatment. The tumours were histologically more atypical and smaller in size, and the patients were older at diagnosis in the 1990's than in the 1980's. In conclusion, the month of treatment, the year of treatment, and the patient's age were found to be independent prognostic predictors in breast cancer, acting as confounders in survival analyses based on the biological prognostic factors.