BACKGROUND AND METHODS. Non-insulin-dependent diabetes mellitus (NIDDM) is a genetically heterogeneous disorder. Maturity-onset diabetes of the young, a form of NIDDM with an early age of onset and autosomal dominant inheritance, can result from mutations in glucokinase, a key enzyme of glucose metabolism in beta cells and the liver. We studied 32 French families with maturity-onset diabetes of the young as well as 21 families with late-onset NIDDM to determine the frequency and clinical features of mutations of glucokinase. Fasting plasma glucose concentrations and oral glucose-tolerance tests were used to determine metabolic status. DNA was isolated from lymphocytes, and DNA polymorphisms in the glucokinase gene were tested for linkage with diabetes. Individual exons of the glucokinase gene from one affected member in each family were amplified by the polymerase chain reaction and screened for mutations by analysis of the conformation-dependent polymorphisms of single-stranded DNA and by DNA sequencing. RESULTS. We found substantial evidence of linkage between the glucokinase locus and maturity-onset diabetes of the young but not between this locus and late-onset NIDDM. Sixteen mutations were identified in 18 of the 32 families with maturity-onset diabetes of the young, but none were found in families with late-onset NIDDM. They included 10 mutations that resulted in an amino acid substitution, 3 that resulted in the synthesis of a truncated protein, and 3 that affected RNA processing. The affected subjects with glucokinase mutations usually had mild hyperglycemia that began during childhood, whereas in subjects with maturity-onset diabetes of the young not due to glucokinase mutations, hyperglycemia usually appeared after puberty. CONCLUSIONS. Mutations in glucokinase are the primary cause of hyperglycemia in a substantial fraction of French patients with maturity-onset diabetes of the young and result in a relatively mild form of NIDDM that can be diagnosed in childhood.