Hepatocyte nuclear factor 4alpha (HNF4alpha) is a nuclear receptor involved in glucose homeostasis and is required for normal beta cell function. Mutations in the HNF4alpha gene are associated with maturity onset diabetes of the young type 1 (MODY1). The aim of the present study was to determine the prevalence and nature of mutations in HNF4alpha gene in Iranian patients with a clinical diagnosis of MODY and their family members. Twelve families including 30 patients with clinically MODY diagnosis and 21 members of their family were examined using PCR-RFLP method and in case of mutation confirmed by sequencing techniques. Fifty age and sex matched subjects with normal fasting blood sugar (FBS) and Glucose tolerance test (GTT) were constituted the control group and investigated in the similar pattern. Single mutation of V255M in the HNF4alpha gene was detected. This known mutation was found in 8 of 30 patients and 3 of 21 individuals in relatives. Fifty healthy control subjects did not show any mutation. Here, it is indicated that the prevalence of HNF4alpha mutation among Iranian patients with clinical MODY is considerable. This mutation was present in 26.6% of our patients, but nothing was found in control group. In the family members, 3 subjects with the age of <or=25 years old carried this mutation. Therefore, holding this mutation in this range of age could be a predisposing factor for developing diabetes in future.