Mutations in the hepatocyte nuclear factor-1-alpha gene cause maturity onset diabetes of the young 3 (MODY 3). Here we describe a new family affected by this disorder carrying the so far unknown mutation Pro224Ser in exon 3. First we identified a 17-year-old patient. OGTT demonstrated that insulin secretion was severely impaired: basal insulin was 3.7 uU/ml and 60 min after an oral glucose load plasma insulin peaked only threefold to 10.7 uU/ml. In addition, this patient carries the homozygous polymorphism Ile27Leu (exon1) in the hepatocyte nuclear factor-1-alpha gene that was shown to be associated with insulin resistance. So far, we have no evidence for insulin resistance in this individual patient. Additionally, two other family members carry the hepatocyte nuclear factor-1-alpha mutation Pro224Ser and the homozygous polymorphism Ile27Leu. A similar case with these two mutations in the HNF-1-alpha gene has not been described before. This data will allow to discover more patients with MODY 3.