(123)I mIBG (meta-iodobenzylguanidine) and (111)In pentetreotide scintigraphy imaging modalities are useful in demonstrating neuroendocrine tumours. Although (111)In pentetreotide is generally held to be a more superior imaging agent than (123)I mIBG for neuroendocrine tumours, we noted a differential uptake of the two agents by different tumour sites within individual patients. In some cases, the two tracers appeared to demonstrate different lesions within the same patient. The aim of this study wasto determine the positivity of the two imaging modalities, the degree of correlation between them and to highlight any clinically useful differences between the two modalities. (123)I mIBG and (111)In pentetreotide images of 149 consecutive, biopsy-proven or biochemically confirmed, neuroendocrine tumour patients were compared. All the patients underwent whole-body imaging and upper abdominal single-photon emission computed tomography (SPECT). The results of both types of imaging were compared, lesion by lesion, for each individual patient. The overall positivity rate for (111)In pentetreotide was 79%, and that for (123)I mIBG was 63%. When both agents were positive, the (111)In pentetreotide highlighted more lesions within the same patient in 33%, whilst the (123)I mIBG highlighted more lesions in 13%. In 12% of patients both agents were positive, but different lesions were seen with the two agents. (111)In pentetreotide has greater positivity than (123)I mIBG for imaging neuroendocrine tumours. However, the two modalities can highlight different tumour lesions, suggesting the presence of phenotypically diverse tumour populations within individual patients. These findings are likely to influence clinical management in the future.