Maturity-onset diabetes of the young (MODY3), a monogenic subtype of non-insulin-dependent diabetes mellitus (NIDDM) with an early age of onset, is characterized by a primary defect in insulin secretion. Recently, it has been shown that mutations of the gene encoding the transcription factor hepatocyte nuclear factor-1α (HNF-1α) cause MODY3. Since NIDDM in Japanese is characterized by insulin secretory defects due to primary β-cell dysfunction, we screened 60 Japanese nonobese subjects with early-onset NIDDM for mutations in this gene, 45 of whom had a first-degree relative with NIDDM. Direct sequencing of the ten exons and flanking introns of the gene in these subjects identified eight nucleotide substitutions including two amino acid changes, Ile-27-Leu and Ser-487-Asn, the frequencies of which were not significantly different in subjects with early-onset NIDDM and nondiabetic subjects. These results suggest that mutations in the HNF-1α gene are not a major cause of early-onset NIDDM in Japanese.