The results of histological studies of operation material from 294 women with breast cancer and immunohistochemical detection of estrogen and progesterone receptors and evaluation of proliferative activity (Ki-67) and expression of proteins p53 and bcl-2 were analyzed. Breast cancer was more incident (60%) in nonindigenous residents of the Sakha Republic (Yakutia) than in representatives of the indigenous population (40% cases). The disease was more incident in women aged 50-59 years (32.6%) and 60-69 years (21.8%) irrespective of the ethnic appurtenance. The findings indicated the most informative criteria of mammary tumor malignancy: low expression of estrogen and progesterone receptors, high expression of Ki-67 and p53, and low expression of bcl-2.