Breast cancer during pregnancy is a rare eventuality. The mean age of women is 34 years. The same histological types (essentially ductal) are found, bur there are more inflammatory forms and/or node involvement and general metastasis. The hormonal receptors are usually negative. The clinical diagnosis is often difficult and delayed. The mammography is perturbed because of breast congestion, giving reason for histological verification. The treatment must consider the fetus as much as possible. The traditional mastectomy associated to axillary nodes dissection is replaced most of the rime by a large tumorectomy with axillary node dissection with the condition not to delay the complementary radiotherapy fore more than three months and to take in consideration the toxicity in relation to the gestational age, the site of impact and the doses given. Also, chemotherapy, if indicated, must begin in the month following the diagnosis. The pronostic is not as good in general as that in the non pregnant woman, because of the delayed diagnosis, the ganglionic metastasis, and the aggravation of the forms accelerated by pregnancy. The prevention consists of a clinical examination of the breast early in pregnancy (particularly after 30 years) and by histologic verification in cases where there is the slightest doubt.