Aims/hypothesis Polymorphisms in the transcription factor 7-like 2 (TCF7L2) gene are associated with type 2 diabetes and reduced insulin secretion. The transcription factor TCF7L2 is an essential factor for glucagon-like peptide-1 (GLP-1) secretion from intestinal L cells. We studied whether a defect in the enteroinsular axis contributes to impaired insulin secretion in carriers of TCF7L2 polymorphisms. Methods We genotyped 1,110 non-diabetic German participants for five single nucleotide polymorphisms in TCF7L2. All participants underwent an OGTT; GLP-1 secretion was measured in 155 participants. In 210 participants, an IVGTT combined with a hyperinsulinaemic-euglycaemic clamp was performed. In another 160 participants from the Netherlands and 73 from Germany, a hyperglycaemic clamp (10 mmol/l) was performed. In 73 German participants this clamp was combined with a GLP-1 infusion and an arginine bolus. Results The OGTT data confirmed that variants in TCF7L2 are associated with reduced insulin secretion. In contrast, insulin secretion induced by an i.v. glucose challenge in the IVGTT and hyperglycaemic clamp was not different between the genotypes. GLP-1 concentrations during the OGTT were not influenced by the TCF7L2 variants. However, GLP-1-infusion combined with a hyperglycaemic clamp showed a significant reduction in GLP-1-induced insulin secretion in carriers of the risk allele in two variants (rs7903146, rs12255372, p<0.02). Conclusions/interpretation Variants of TCF7L2 specifically impair GLP-1-induced insulin secretion. This seems to be rather the result of a functional defect in the GLP-1 signalling in beta cells than a reduction in GLP-1 secretion. This defect might explain the impaired insulin secretion in carriers of the risk alleles and confers the increased risk of type 2 diabetes.