MODY3, linked to mutations in the HNF1[alfa] gene is an autosomal dominant form of diabetes. The knowledge about the molecular background of this disease has a wide impact on diabetic research, as well as on prophylaxis, prognosis, and on the clinical practice. The aims were: to recruit the families to screen for MODY3 mutations, to analyse the segregation and to provide the characteristics of the mutations, to characterize mutation carriers, and to search for developmental renal abnormalities. Mutation screening was performed by direct sequencing. The clinical characteristics included the data concerning the diabetes, its chronic complications, coexisting abnormalities, antropometric measurements, and biochemical parameters. The mutation carriers were compared to a group of diabetic subjects with typical type 2 diabetes and non-diabetic volunteers. The search for renal malformation was performed by USG examination. 47 probands were examined, 13 MODY3 families were identified. There were 2 novel mutations Ser249Pro and Asn257Thr. Amongg 56 identified HNF1[alfa] mutation carriers there were 46 diabetic patients. In MODY3 group there was a high prevalence of chronic complications, similar to type 2 diabetes group. The frequency of the metabolic syndrome components was lower than in type 2 diabetic group. In 5 HNF1[alfa] mutation carriers from 3 families the renal malformations were identified. This study is the first large-scale search for MODY3 mutations in a Polish population. It is also one of a few ones in the world in which diabetes complications were examined in MODY patients and the first description of the association of renal malformation with the HNF1[alfa] gene mutations.