Adult lactase deficiency – the inability to effectively digest lactose in adulthood – causes digestive discomfort and may be associated with the elimination of a whole category of dairy products from the diet. The objective of the study was to assess the prevalence of the gene associated with lactose deficiency for a sample of 24 thousand people living in Russia. It turned out that among the studied population groups, the frequency of occurrence of the GG genotype in rs4988235 was higher than the average in European populations. In particular, the prevalence of the genotype of lactase deficiency in the group of Eastern Slavs was 42.8% (95% CI: 42.1–43.4%). The regional prevalence of lactase deficiency was also investigated depending on the place of residence.