Comparison of the incidence of breast cancer (BC) in different ethnic groups by periods revealed a significant difference in the rates between the native and alien populations in both the whole republic and in Ulan-Ude and rural areas (p<0.05). In Ulan-Ude, the incidence of BC for the indigenous population (22.2±1.20/0000) is 1.9 times lower than that for the newcomers (41.7±2.40/0000), with a significant difference of 2.8 times compared with that for the rural population (20.4±1.7 and 57.5±3.20/0000, respectively). The higher the proportion of newcomers and retirement-aged people, the higher the risk of BC; and, conversely, the decline in the proportion of these population groups is accompanied by a reduction in the risk of BC. The BC mortality rates for the alien population of the Republic of Buryatia (16.8±1.60/0000) are 2.3 times higher than those for the indigenous population (7.3±0.90/0000). The difference is statistically significant (p<0.05).