Background/Aims: Adult-type hypolactasia (primary lactose malabsorption) affects most of world’s human population and limits the use of fresh milk due to lactose intolerance. The diagnosis of adult-type hypolactasia has been difficult to establish because of unsatisfactory diagnostic methods. C/T 13910 single nucleotide polymorphism residing 13910 base pairs from the 5′end of the lactase gene has been shown to be associated with lactase persistence. The aim of the study was to assess the applicability of the C/T-13910 variant as a diagnostic test for adult-type hypolactasia during childhood. Methods: Intestinal biopsies were obtained from 329 children and adolescents of African, Finnish, and other White origins aged 0.1-20 years undergoing upper gastrointestinal endoscopy because of abdominal complaints. The biopsies were assayed for lactase, sucrase, and maltase activity and genotyped for the C/T-13910 variant using polymerase chain reaction minisequencing. Results: The frequency of the C/C-13910 genotype defining lactase non-persistence was well in agreement in this study with published figures for the prevalences of adult-type hypolactasia in Africans and Whites. The C/C-13910 genotype was associated with very low lactase activity (<10 U/g protein) in the majority of children tested at 8 years of age and in every child older than 12 years of age giving a specificity of 100% and sensitivity of 93% for the genetic test. The decline of lactase activity was somewhat earlier in African compared with Finnish children with C/C -13910 genotype (p < 0.03). Conclu sions: Genetic test of C/T -13910 polymorphism can be used as a first stage scr eening test for adult-type hypolactasia. Background / Aims: Adult-type hypolactasia (primary lactose malabsorption) affects most of world’s human population and limits the use of fresh milk due to lactose intolerance. The diagnosis of adult-type hypolactasia has been difficult to establish because of unsatisfactory diagnostic methods. C / T 13910 single nucleotide polymorphism residing 13910 base pairs from the 5’end of the lactase gene has been shown to be associated with lactase persistence. The aim of the study was to assess the applicability of the C / T-13910 variant as a diagnostic Test for adult-type hypolactasia during childhood. Methods: Intestinal biopsies were obtained from 329 children and adolescents of African, Finnish, and other White origins aged 0.1-20 years undergoing upper gastrointestinal endoscopy because of abdominal complaints. The biopsies were assayed for lactase, sucrase, and maltase activity and genotyped for the C / T-13910 variant using polymerase chain reaction minisequencing. Results: The frequency of the C / C-13910 genotype defining lactase non-persistence was well in agreement in this study with published figures for the prevalences of adult-type hypolactasia in Africans and Whites. The C / C-13910 genotype was associated with very low lactase activity ( <10 U / g protein) in the majority of children tested at 8 years of age and in every child older than 12 years of age giving a specificity of 100% and sensitivity of 93% for the genetic test. The decline of lactase activity was somewhat earlier in African compared with Finnish children with C / C-13910 genotype (p <0.03). Conclu sions: Genetic test of C / T-13910 polymorphism can be used as a first stage scr eening test for adult-type hypolactasia.