Recent studies have shown that mutations in the transcription factor hepatocyte nuclear factor (HNF)-1α are the cause of one form of maturity-onset diabetes of the young (MODY3). These studies have identified mutations in the mRNA and protein coding regions of this gene that result in the synthesis of an abnormal mRNA or protein. Here, we report an Italian family in which an A→C substitution at nucleotide -58 of the promoter region of the HNF-1α gene cosegregates with MODY. This mutation is located in a highly conserved region of the promoter and disrupts the binding site for the transcription factor HNF-4α, mutations in the gene encoding HNF-4α being another cause of MODY (MODY1). This result demonstrates that decreased levels of HNF-1α per se can cause MODY. Moreover, it indicates that both the promoter and coding regions of the HNF-1α gene should be screened for mutations in subjects thought to have MODY because of mutations in this gene.