Mutations in the hepatocyte nuclear factor–1α (HNF-1α) gene have recently been shown to cause maturityonset diabetes of the young (MODY). We have examined 15 U.K. MODY families for mutations in the coding region of the HNF-1α gene. Eight different mutations, three frameshift (P291fsinsC, P379fsdelCT, and A443fsdelCA) and five missense mutations (P129T, R131W, R159W, P519L, and T620I), were identified in eleven families (73%). The previously reported mutation P291fsinsC was found in four pedigrees. A screen of a further 32 probands with early onset (<40 years of age) NIDDM showed the mutation in two additional families. This common mutation was present on at least three different haplotypes, suggesting that its high frequency is due to recurrent mutation rather than a founder effect. We have demonstrated that mutations in the HNF-1α gene are a common cause of MODY in U.K. families and result in early onset NIDDM with a progressive clinical course. Mutation-based genetic counseling can now be considered for the majority of patients with MODY.