The complete protocol for the treatment of breast cancer at the Institute Gustave Roussy, Villejuif, France, is presented. Treatment is carried out by a multidisciplinary committee dealing with nearly one thousand new breast cancers annually. The protocol is based on initial classification by the T.N.M. system and by characteristics indicating "aggressive biological behaviour". Patients are separated into three broad groups for which the mainstays of treatment are surgery, radiotherapy, and chemotherapy. The rationale and results of the protocol are presented.