As mammography is increasingly prescribed systematically and mammography films are of better quality, diagnostic biopsies are performed in growing numbers, leading to the discovery of "borderline" histological lesions which were relatively rare a few years ago. The authors have studied the frequency and clinical presentation of these lesions. In patients with atypical hyperplasia simple monitoring is the rule since the short- and long-term effects of medical treatments are too uncertain for these to be recommended routinely. The finding at histology of lobular carcinoma in situ is regarded as indicating a high risk of cancer infiltrating both breasts, and in all but some special cases close supervision with 6-monthly clinical examination and annual mammography is advocated. Whether or not endocrine treatments reduce the incidence of infiltrating carcinoma is unknown. In patients with intraductal carcinoma mastectomy gives excellent results at the cost of a considerable loss of tissue. In view of the N.S.A.B.P. and Curie Institute experience, tumorectomy alone is not recommended because of the high risk of recurrence, but a conservative treatment (limited surgery followed by radiation) is permitted. Conservative treatment may also be attempted in Paget's disease without palpable tumour.