Purpose: The identification of multiple tumors in the breast is associated with increased nodal involvement when compared with similar staged unifocal disease. This study compares two methods of tumor size assessment to predict tumor behavior in the relationship between size and axillary node involvement for patients with multifocal and multicentric breast cancer. Methods: The histologic reports of every patient with multifocal breast cancer treated in New South Wales between April 1995 and September 1995 were examined. Tumors were assessed using two size estimates: (1) largest tumor focus diameter and (2) the aggregate diameters of all tumor foci. The dimensions were compared with unifocal tumors and against node positivity. Results: Ninety-four (11.1%) of 848 women had multifocal breast cancer and of these 49 women (52.1%) had axillary node involvement compared with 37.5% with unifocal breast cancer (P =.007). The use of aggregate dimension reclassified significant numbers of multifocal tumors at a more advanced stage. Use of this method to stage cancers, rather than the largest tumor size, removed the excess node positivity when compared with unifocal, stage-matched breast carcinomas. Conclusion: The tendency of breast tumors to metastasize is a reflection of the total tumor load. Failure to measure the additional tumor burden provided by multiple small foci may understage a woman's disease. This may deny patients the opportunity of adjuvant therapies if the contribution of the smaller foci to the incidence of node positivity and survival is ignored.