MODY 3 belongs to monogenic forms of diabetes mellitus and is caused by monoallelic mutation in gene for transcription factor HNF-1alpha, essential for regulation of beta-cell function. Clinical presentation of MODY 3 is similar to that of type 1 diabetes. Although MODY 3 patients are not threatened by ketoacidosis, tight metabolic control is important for prevention of chronic diabetic complications. In the sibbling pair diabetes was manifested by osmotic symptoms resulting from hyperglycaemia at the age of 18 years (brother) resp. 15 years (sister) and both of them started being treated with intensified insulin treatment. Metabolic control of the brother was very tight with HbA1c 3.3 % but frequent hypoglycaemias occured. On the contrary metabolic control of the sister was very poor due to her non-compliance (HbA1c 10.9 %, IFCC). Molecular-genetic testing proved HNF-1alpha gene mutation (Arg200Gly). In accordance with the references treatment with sulphonylurea derivate glibenclamide was initiated [at the doses 1.25 (brother) resp. 7.5 (sister) mg/day] and insulin treatment was discontinued. The treatment change led to better quality of life and metabolic control in both the patients and suprisingly to the lower frequency of the hypoglycaemias in the brother (HbA1c decreased from 3.3 % to 2.8 % in three months in the brother resp. from 10.9 % to 10.0 % in two months in the sister). Molecular-genetic testing enables the change of treatment leading to better quality of life and metabolic control, although its longterm safety and efficacy will have to be confirmed.