A retrospective study was carried out to determine the clinical significance of local recurrence after simple mastectomy and node biopsy for primary operable breast cancer, without postoperative irradiation or systemic adjuvant therapy. Local recurrence was defined as a histologically proven lesion in or deep to the mastectomy skin flaps. A total of 966 patients with a median follow-up of 7 years were reviewed. Of these, 223 (23 per cent) developed local recurrence but half the tumours were small single lesions; 70 women had multiple discrete lesions and 21 diffuse carcinomatous dermal infiltration. Local recurrence showed significant associations with tumour grade, nodal status and the presence of lymphovascular invasion in the primary tumour. A predictive index containing these three variables was constructed. Adjuvant irradiation of the flaps is recommended for patients with high scores; such women would otherwise have a 39 per cent chance of developing local recurrence by 5 years. Different types of local recurrence have different chances of responding to local therapy: 13 per cent of single local recurrences, 32 per cent of multiple spot recurrences and 70 per cent of the diffuse type failed to respond to local therapy. Local recurrence predicts reduced patient survival.