A 52-year-old woman with early-onset diabetes mellitus, severe diabetic retinopathy, and a family history of a dominant inheritance pattern was referred to our hospital. Direct sequencing revealed a novel mutation in the HNF-4alpha gene (R244Q). The position of the mutation in the amino-acid sequence of the gene is well conserved among species and transcriptional activity of the mutant gene was significantly reduced. Therefore, a diagnosis of maturity-onset diabetes of the young (MODY)-1 was made. Genetic testing enabled early diagnosis of diabetes in the patient's 20-year-old daughter, which we consider to be important for the prevention of diabetic complications.