Heterozygous inactivating mutations in the glucokinase gene cause the mildest form of maturity-onset diabetes of the adolescents. However, homozygous or compound heterozygous mutations in the glucokinase gene are a rare cause of permanent neonatal diabetes mellitus. Herein, we present the case of a male child with permanent neonatal diabetes mellitus whose mutational analysis revealed a novel homozygous deletion mutation in the glucokinase gene. The male proband of Turkish ancestry from consanguineous parents was born at 37 weeks gestation with a birth weight of 1870 g (<3rd percentile). Hyperglycemia developed during the first postnatal day and diabetes-related autoantibodies were negative. He was put on insulin on the first day of life. Insulin has never been discontinued since then. The mother was aged 35 years and had gestational diabetes. The father and the two brothers had impaired fasting glucose. Both parents and brothers were heterozygous for this mutation.