Breast tumour of pregnancy ('lactating' adenoma).

Fourteen cases of a benign lesion of the breast, often referred to as 'lactating' adenoma, are described. They were all first noticed during pregnancy. The microscopic changes are similar to those seen in the normal pregnant breast but vary in degree and are out of phase with it. Any association with fibroadenoma or fibrocystic disease seems coincidental. 'Lactating' adenomas are also clearly distinguishable from tubular adenomas, which consist of masses of closely set tubules, like ductules of the normal resting breast, and are not related to pregnancy. Immunocytochemistry using antisera to breast and tumour associated antigens can demonstrate clear differences between 'lactating' adenoma, other breast lesions, and the normal pregnant breast. A plea is made for the designation of these lesions as 'breast tumour of pregnancy' as they do not arise in the lactating period.