Purpose: The objective of this study is to determine whether the aggregate tumour size of every focus in multifocal breast cancer more accurately predicts 10-year survival than current staging systems which use the largest or dominant tumour size. Patients and methods: This study examined the original histological reports of 848 consecutive patients with invasive breast cancer treated in New South Wales (NSW), Australia between 1 April 1995 and 30 September 1995. Multifocal tumours were assessed using two estimates of pathologic tumour size: largest tumour focus diameter and the aggregate diameter of every tumour focus. The 10-year survival of patients with multifocal tumours measured in both ways was compared to that with unifocal tumours. Results: At a median follow-up of 10.4 years, 27 of 94 patients (28.7%) with multifocal breast cancer have died of breast cancer compared to 141 of 754 (18.7%) with unifocal breast cancer (P = .022). Ten-year survival was not affected by size for tumours measuring 20 mm or less, whether or not dominant tumour size (87.9%) or aggregate tumour size (87.0%) was used for multifocal tumours, compared to unifocal tumours (88.1%). For tumours larger than 20 mm, 10-year survival was 72.1% for unifocal tumours compared to 54.7% (P = .008) for multifocal tumours using dominant tumour size, but this was 69.5% and not significant when multifocal tumours were classified using aggregate tumour size (P = .49). Multivariate analysis also confirmed the above-mentioned results after adjustment for important prognostic factors. Conclusion: Aggregate size of every focus should be considered along with other prognostic factors for metastasis when treatment is planned. The current convention of using the largest (dominant) lesion as a measure of stage and associated breast cancer survival needs further validation.