Use of large (6 in. X 4 in.) paraffin sections in the routine examination of specimens from patients with operable breast cancer has revealed multiple quadrant involvement by carcinoma in 20% of 186 consecutive cases. Eighty-nine per cent of patients had invasive carcinomas, which were associated with carcinoma in situ (CIS) in 71%. CIS was frequently extensive, and in 47% was remote from the principal invasive lesion. Fourteen patients had multifocal invasive disease with 34 separate primary tumours. The involvement of 32% of sampled axillary nodes by tumour, combined with this significant incidence of multiple quadrant disease, brings into question the rationale of local surgery in most cases even of early breast cancer. Mastectomy with node sampling permits full assessment of the extent of disease, which is essential for the logical selection of patients for adjuvant therapy. The potential role for limited surgery in non-invasive and very small invasive tumours in combination with radiotherapy remains under evaluation.