In 1048 breast cancer patients, operated in the period 1969-1985 at the University of Erlangen Clinic of Obstetrics and Gynecology, estimations were made to determine the relative contribution of mammography and meticulous histology to the diagnosis of simultaneous contralateral cancer. The incidence of a bilateral simultaneous disease was 17% (6.1% invasive forms, 10.6% in situ). Complete histological examination of the extirpated tissue as well as the occurrence of discrete radiological signs could account for the detection of 41% of all invasive and 39% of all in situ forms respectively. Those contralateral breast cancer cases detected just by means of mammography, but without any presence of clinical signs, recorded an average diameter of 9 mm and in 20% had metastacised to the axillary lymph nodes. Comparatively, clinically and radiologically diagnosed cases were 17 mm on average and the occurrence of axillary lymph node involvement was 41%. The conclusion is drawn, that a meticulous diagnostic effort is necessary in view of the high incidence of occurrence of simultaneous cancer on the other breast and the prognostic importance of an early diagnosis for many patients. Every minute radiologically detected sign in the other breast of patients with mammary carcinoma requires careful diagnostic clarification. However, it should be considered that the data presented were gathered in a situation in which the radiologist performed the X-ray examination, knowing that an excision would have been carried out in 85 percent of all cases even without his specific localisation. The presence of additional risk factors (lobular cancer, multicentricity and family history of breast cancer) make such an effort justified and obligatory as well.