PIP: Incidence of mortality caused by breast cancer goes from 25-30/100,000 in Canada, U.S. and Great Britain, to 3-4/100,000 in Japan. Incidence of morbidity is 74/100,000 in the U.S. 62.5/100,000 in Europe, and 85/100,000 in Sweden. This paper investigates the possible risk factors influencing breast cancer. Several authors have found a genetic predisposition to breast cancer; daughters of mothers with breast cancer have 27-30% more probability of getting the disease; a high percentage of cases with family antecedents, 35.8%, is diagnosed before menopause. According to some authors early menopause therapeutically induced has a protective effect against breast cancer; according to other authors estriol can modify the carcinogenic effects of other estrogens; thus, women with low levels of estriol have a higher risk of breast cancer. Level of plasmatic hormonal concentrations may also play a role; a diet poor in iodine may be an added risk. Other risk factors seem to be age under 13 at menarche, late menopause, low fertility especially after 35, absence of breastfeeding, height over 165 cm, weight above 70 kg, and a diet rich in animal fats. There is no proven association between oral contraceptive use and increased incidence of breast cancer.