Maturity-onset diabetes of the young (MODY) is a form of non-insulin-dependent diabetes mellitus characterized by an early age of onset, usually before 25 years of age, and an autosomal dominant mode of inheritance. The largest and best-studied MODY pedigree is the RW family. The majority of the diabetic subjects in this pedigree has a reduced and delayed insulin-secretory response to glucose, and it has been proposed that this abnormal response is the manifestation of the basic genetic defect that leads to diabetes. Using DNA from members of the RW family, we tested more than 75 DNA markers for linkage with MODY. A DNA polymorphism in the adenosine deaminase gene (ADA) on the long arm of chromosome 20 was found to cosegregate with MODY. The maximum logarithm of odds (lod score) for linkage between MODY and ADA was 5.25 at a recombination fraction of 0.00. These results indicate that the odds are greater than 178,000:1 that the gene responsible for MODY in this family is tightly linked to the ADA gene on chromosome 20q.