100 cases (2.77 per cent) of bilateral mammary cancer (simultaneous occurrence in 26 and consecutive in 74) were observed out of 3600 patients hospitalized for mammary cancer. Operability was considered to be the most suitable criterio for the recognition of the primary tumour. Numerous anamnestic factors have been considered in order to explain the prevalence of bilateral mammary cancer in some patients, but no convincing evidence was reached. Patients affected by bilateral cancer showed, in comparison with other groups, a more prolonged genital life. It was furthermore observed that: 1) the site of occurence in the different breast quadrants is unrelated to the classical routes of lymphatic spread; 2) patients with invasion of lymphnodes of the internal mammary chain show a much more severe prognosis; 3) five-year results do not differ in patients with bilateral or unilateral breast cancer; they are rather dependent upon invasion of regional nodes. Personal and literature results show that the so-called prophylactic removal of the healthy breast does not improve results after 5 years.