Lactase-phlorizin hydrolase (LCT), more commonly known as lactase, is an enzyme responsible for cleaving lactose into absorbable monosaccharides, glucose and galactose. LCT deficiency (hypolactasia – HL) is caused by a decreased activity of LCT in the small intestinal villi and potentially results in lactose malabsorption what may lead to the development of clinical symptoms (diarrhea, bloating, flatulence and cramps) andavoiding milk products in the diet. HL is the world’s most common enzyme deficiency in humans. HL exists in three distinct forms – congenital, primary and secondary. Adult type hypolactasia (ATH) is the most common phenotype found in human. It is a genetically predetermined physiological condition inherited through an autosomal recessive mode which results in a decline of lactase activity after weaning. ATH is associated with the LCT -13910 C>T polymorphism worldwide, except in Africa. Lactase non-persistence has been observed in individuals with the C/C-13910 genotype, whereas lactase persistence in subjects with remaining allelic variants. Small intestine biopsy is the only diagnostic procedure allowing for the direct measurement of LCT activity, however due to its invasive nature it is hardly accepted by patients. Therefore, LCT status is often inferred simply by assessing the patient’s lactose digestion. A lactose tolerance test can be performed after lactose load and then measuring blood glucose concentration or breath hydrogen (preferably hydrogen and methane) expiration. A genetic test of the C/T-13910 polymorphism is also available at present. It is a reliable method in excluding/confirming ATH predisposition. However, it definitely does not assess lactose tolerance or malabsorption.