A prospective study of the axillary nodal status of women found to have an invasive breast cancer within the prevalence screening round is reported. Thirty-one per cent of patients were lymph node positive. Twenty-two per cent of patients with an impalpable tumour and 41% of those with a palpable tumour had involved axillary lymph nodes. Only 6% of patients with a tumour of less than 10 mm had diseased axillary nodes. Of the factors examined only the size of the invasive component of the primary tumour was related to the axillary lymph node status. This proportion of patients with positive lymph nodes is higher than might be expected and is likely to represent those patients with potentially symptomatic breast tumours detected by breast screening in the first round.