Axillary lymph node dissection (ALND) may not be necessary in women with breast cancer who have micrometastasis in a sentinel node (SN), owing to the low risk of non-SN (NSN) involvement. The aim of this study was to identify a subgroup of women with a micrometastatic SN and a negligible risk of positive NSNs in whom ALND may be avoided. Some 237 of 241 women with a macrometastatic SN and 122 of 138 with a micrometastatic SN underwent completion ALND and were compared with respect to NSN involvement. The 122 patients with SN micrometastasis were further analysed to determine factors that could predict the risk of positive NSNs. A total of 121 (51.1 per cent) of 237 women with SN macrometastasis had positive NSNs compared with 22 (18.0 per cent) of 122 with SN micrometastasis (P < 0.001). Multivariate analysis showed that size of SN micrometastasis (odds ratio 3.49 (95 per cent confidence interval (c.i.) 1.32 to 9.23); P = 0.012) and presence of lymphovascular invasion (odds ratio 0.23 (95 per cent c.i. 0.05 to 1.00); P = 0.050) were significantly associated with positive NSNs. SN micrometastasis less than 0.5 mm in diameter combined with absence of lymphovascular invasion was associated with an 8.5 per cent risk of NSN involvement. Size of micrometastasis and presence of lymphovascular invasion were significantly related to the risk of finding additional positive axillary lymph nodes when the SN contained only micrometastasis. Copyright (c) 2005 British Journal of Surgery Society Ltd. Published by John Wiley & Sons, Ltd.