Abstract: Introduction and aim: To assess the prevalence of lactase gene promoter C/T13910 phenotypes in patients with lactose intolerance symptoms and to analyze the birth-cohort phenomenon in lactose-intolerant patients. Method: 1266 patients consulted between 2010 and 2017 were enrolled. Five-year cohort periods of patients born before 1939 and after 1995 were defined and the prevalence of C/T13910 phenotypes was calculated. C/T13910 phenotypes were determined by polymerase chain reaction. Results: The prevalence of the CC phenotype was 43.1%, TC was detected in 44.3% and TT in 12.6% of the cases. The prevalence of the CC phenotype showed a non-linear incremental increase in females, from 9.13% in those born before 1939 to 37.7% in those born after 1995. Conclusion: The prevalence of CC phenotype raised incrementally in females, suggesting a gender/hormonal influence. Orv Hetil. 2019; 160(14): 549–554.