In many patients, the sentinel lymph node (SLN) is the sole site of regional nodal metastasis. This subgroup of patients would not be expected to benefit from completion axillary lymph node dissection (CALND). This study evaluated the factors that may determine the likelihood of additional positive nodes in the axilla in the presence of sentinel node metastasis. A total of 618 breast cancer patients underwent SLN biopsy based on lymphoscintigraphy, intraoperative gamma probe detection, and blue dye mapping using 99mTc-nanocolloid and Patent Blue V injected peritumourally. This was followed by standard axillary node clearance at the same operation. Of the 201 patients with a positive SLN, 105 (52%) patients had no further positive nodes in the axilla, 96 (48%) patients had additional metastasis in non-sentinel lymph nodes (NSLN) upon CALND. In patients with a positive SLN, increasing tumour size and tumour grade significantly increased the frequency of additional positive nodes on univariate analysis. The number of SLNs removed and the number of negative SLNs were significant negative predictors. Increasing tumour burden in the sentinel nodes (determined by the number of positive SLNs) was significantly associated with increasing likelihood of positive NSLNs. Multivariate analysis revealed that the rest of the axilla is more likely to be positive if there are more positive than negative SLNs removed and more likely to be negative otherwise. A group of cases from one centre (Cardiff) were subjected to further detailed analysis. Tumour burden in the positive SLN was assessed by measuring the size of metastasis, percentage replacement of the SLN by tumour and by documenting extracapsular extension (ECE) around the SLN. Of the 64 patients with a positive SLN, 34 (53%) patients had no further positive nodes in the axilla, 30 patients (47%) had additional metastasis in NSLNs upon CALND. Increasing tumour burden in the SLN was associated with additional positive nodes in the axilla. Multivariate analysis revealed that size of the SLN metastasis is the most important predictor of involvement of only the SLN. Overall, in patients with a positive SLN, the difference in the number of positive and negative SLNs removed and size of the metastasis in the SLN, all predicted the frequency of additional positive nodes.