Maturity onset diabetes of the young (MODY) is a genetically heterogeneous form of type 2 diabetes that is characterized by autosomal dominant inheritance, onset in early adulthood and a primary defect in insulin secretion. Mutations in at least six genes have been shown to underlie MODY, including mutations in GCK (encoding glucokinase, also called MODY2) and mutations in HNF1A (encoding hepatocyte nuclear factor-1alpha, also called MODY3). We sequenced genomic DNA from probands of seven Canadian MODY families. In four probands, we detected four novel GCK mutations, namely IVS2-7G>A, G72R, T206R and S263P. In three other probands, we detected three HNF1A mutations, of which two were novel, namely 1051delCA and Q250X, and one had been previously reported, namely R131Q. The novel mutations expand the spectrum of MODY mutations. In addition, knowledge of the specific defect can be used to pre-symptomatically identify family members at risk for developing MODY.