Lactose intolerance is found in 70% of the world-wide population. Lactase activity gradually increases till birth and then, after the early months of life, it physiologically begins to lessen. In about 30% of the population it remains active also as an adult due to a mutation: in Europe a substitution of C with T in position 13910 of the lactase gene. Our data show that CC genotype, associated with lactase non-persistence, is found in type 1 diabetes with a high prevalence of 67%. To evaluate the “glycaemic-metabolic control” we investigated the glycosylated haemoglobin (HbA1c) values and we noted that there is no significant difference between CC (non-absorber) and CT/TT (absorber) genotypes of diabetic patients. Glutamic acid decarboxylase antibodies (GADA) are positive in 58% of diabetic patients with CC genotype: this presence could be linked to the intestinal membrane hyperpermeability.