Serial sectioning of ipsilateral axillary lymph nodes judged to be disease-free after routine histological examination revealed micrometastases in 83 (9%) of 921 breast cancer subjects. These patients had a poorer disease-free (p = 0.003) and overall (p = 0.002) survival after 5 years' median follow-up than did patients whose nodes remained negative on serial sectioning. The presence of axillary lymph node micrometastases correlated with the presence of vascular invasion and tumour size (p less than 0.0001 and p = 0.02, respectively). The presence of occult micrometastases remained statistically significant after adjusting for other prognostic factors. The detection of these micrometastases in lymph nodes may identify a high risk "node-negative" population and should be considered as part of the routine pathology examination.