We present a unique case of a 19-year-old man with a positive family history of persistent mild hyperglycemia and a novel V84I mutation in ABCC8. The proband was initially detected to have fasting hyperglycemia (ranging 6.1-6.4 mmol/L) at the age of 12 years. Increased fasting blood glucose was also subsequently detected in five additional family members (in his twin brother, sister, mother, maternal aunt, and grandfather). The grandfather has been known to have mild diabetes since 30 years and has never been treated. After having excluded a causative mutation in five maturity-onset diabetes of the young genes (MODY1-4 and 6), we identified a novel ABCC8 V84I mutation, which segregated with autosomal dominant transmission of mild hyperglycemia within three generations. This mutation that is located in a conserved area of transmembrane domain TMD0 seems to be a rare cause of clinical phenotype resembling glucokinase-deficient diabetes.