204 pts with breast cancer were compared to 208 pts of similar age, with pathologies other than cancer, to assess the frequency of biological and social factors related to breast cancer, in Chilean females. The pts with breast cancer were in the 40 to 65 year old group, had a higher frequency of AB blood group, lower pregnancy and delivery rates and later pregnancies. Age of menarche and menopause, prolonged breast feeding, oral contraceptives and nutritional state were not related to a higher frequency of breast cancer. Further studies that include the analysis of previous breast pathology, diabetes, psychosomatic factors, alcohol and caffeine consumption, tobacco and others are suggested.