Maturity-onset diabetes of the young (MODY) is a subtype of type 2 diabetes that presents from the second decade and has an autosomal dominant mode of inheritance. We have investigated the glucokinase gene, a candidate gene for diabetes, in two MODY pedigrees. In a large 5-generation pedigree (BX) with 15 diabetic members, use of a microsatellite polymorphism revealed linkage of diabetes to the glucokinase locus on chromosome 7p. A peak lod score of 4.60 was obtained at a recombination fraction (theta) of zero. This finding suggests that a defective glucokinase gene contributes to the diabetes phenotype in this pedigree. This is not universal in MODY since linkage to the glucokinase locus was excluded in a second pedigree M (lod score = -7.36 at theta = 0). The affected members in pedigree BX were diagnosed either when young (in pregnancy or on screening) or when they presented symptomatically in middle and old age; most of them were treated by diet alone. Defects in the glucokinase gene may play an important part in the pathogenesis of type 2 diabetes.