Aims/hypothesis A reliable method for in vivo quantification of pancreatic beta cell mass (BCM) could lead to further insight into the pathophysiology of diabetes. The glucagon-like peptide 1 receptor, abundantly expressed on beta cells, may be a suitable target for imaging. We investigated the potential of radiotracer imaging with the GLP-1 analogue exendin labelled with indium-111 for determination of BCM in vivo in a rodent model of beta cell loss and in patients with type 1 diabetes and healthy individuals. Methods The targeting of In-111-labelled exendin was examined in a rat model of alloxan-induced beta cell loss. Rats were injected with 15 MBq In-111-labelled exendin and single photon emission computed tomography (SPECT) acquisition was performed 1 h post injection, followed by dissection, biodistribution and ex vivo autoradiography studies of pancreatic sections. BCM was determined by morphometric analysis after staining with an anti-insulin antibody. For clinical evaluation SPECTwas acquired 4, 24 and 48 h after injection of 150MBq In-111-labelled exendin in five patients with type 1 diabetes and five healthy individuals. The tracer uptake was determined by quantitative analysis of the SPECT images. Results In rats, In-111-labelled exendin specifically targets the beta cells and pancreatic uptake is highly correlated with BCM. In humans, the pancreas was visible in SPECT images and the pancreatic uptake showed high interindividual variation with a substantially lower uptake in patients with type 1 diabetes. Conclusions/interpretation These studies indicate that (111)Inlabelled exendin may be suitable for non-invasive quantification of BCM.