Breast cancer treatment trials from the US National Surgical Adjuvant Breast and Bowel Project have established breast-conserving operations as a replacement for radical mastectomy (NSABP B-04), and have shown that in terms of survival free from distant disease there was no significant difference between lumpectomy, lumpectomy plus breast irradiation, and total mastectomy (NSABP B-06). 9-year follow-up data from B-06 are used here to address the issue of ipsilateral breast tumour recurrence (IBTR) and the development of distant disease, a question with important clinical and biological implications. A Cox regression model on fixed co-variates (ie, features such as tumour type or size present at surgery and not subsequently alterable) and on IBTR, which is time dependent and not fixed, revealed that the risk of distant disease was 3.41 times greater after adjustment for co-variates in patients in whom an IBTR developed. IBTR proved to be a powerful independent predictor of distant disease. However, it is a marker of risk for, not a cause of, distant metastasis. While mastectomy or breast irradiation following lumpectomy prevent expression of the marker they do not lower the risk of distant disease. These findings further justify the use of lumpectomy.