Bilateral breast cancer. A second primary breast cancer in the opposite breast can be either synchronous or metachronous. The majority are metachronous. A woman who has had breast cancer has a fivefold increase in risk for a second breast cancer. Additional risk factors include multifocal cancer, lobular carcinoma in situ, and an original cancer at an early age with long survival. Lobular carcinoma in situ is predominantly a marker for the subsequent development of a second primary breast cancer. The incidence of synchronous bilateral cancer is approximately 1% to 2% and that of metachronous cancer 5% to 6%. The cancer can be invasive or noninvasive. Mammography has increased the number of synchronous cancers found but not the overall incidence. The incidence of invasive cancer detected by random biopsy of the opposite breast is not high enough to justify routine adoption of this procedure. The remaining breast must be followed for the remainder of the patient's life by physical examination and annual mammography. The treatment of the secondary primary breast cancer should be that appropriate for the stage of the disease. The prognosis for the woman with a second primary breast cancer is quite favorable and is dependent on the stage of both the first and the second cancer. 