This study of 786 node-negative breast cancer patients demonstrates that the prognostic effect of age at operation, tumor diameter and mean nuclear area (MNA) changed over time. Identifying patients at higher risk of dying after 5 years may be of clinical importance. These are patients who may have had a low metastatic tumor burden at the time of operation and in whom cytostatic adjuvant treatment could be of value. When estimating 10-year survival, a significant loss of strength was found for age. A near significant loss of effect was also found for MNA, while tumor diameter was the variable best maintaining prognostic power during the observation period. Due to the loss of strength for MNA and age when estimating 10-year survival, the observation period was divided into two periods. When estimating 5-year survival, MNA was the strongest variable, while tumor diameter and age gave additional significant information. When estimating the next 5 years, excluding patients who died before 5 years, only tumor diameter gave significant prognostic information (573 cases). This again stresses the importance of tumor diameter in assessing prognosis in node-negative patients.