Monogenic, autosomal dominant GCK-MODY is related to the mutation of the enzyme regulating insulin secretion – glucokinase. The disease has a mild course, hyperglycemia does not progress, the risk of chronic diabetes complications is low and farmacological treatment is usually not necessary. Proper diet is sufficient to keep good metabolic control.The aim of the study was to investigate the molecular background and some clinical features of GCK-MODY insouth-eastern Poland. In 42 identified GCK-MODY families 27 different mutations were found, among them 5 novel mutations, not described in other populations. The group of GCK gene mutation carriers was characterised by anthropometric and biochemical parameteres, comorbobidties, diabetes complications and methods of treatment. This group was compared to the group of healthy subjects. The high rate of mutations’ penetration was confirmed in the analysed group of GCK-MODY patients. The patients were not obese and in majority of cases the features of metabolic syndrome were absent. The incidence of the hypertension did not differ from the healthy control group. Severe chronic diabetes complications were absent. After the termination of the study medical data of the patients was reanalyzed and the influence of molecular diagnosis on therapeutic proceedings, clinical picture and metabolic control was assessed. All patients treated with insulin stopped this treatment. The number of patients treated with oral hypoglyceamic agents decreased and the number of patients on diet – increased. This change of therapeutic approach didn’t influence the metabolic control of diabetes.