Recent significant advances in understanding the biology of gastrointestinal stromal tumours (GIST) have led to the introduction of a new targeted therapy (imatinib mesylate, Glivec). Hopes of a new era of a specific cancer therapy, however, have been tempered by the recognition that a significant proportion of patients who initially respond to the drug eventually become resistant to it. Given the successful development of peptide receptor scintigraphy and radiotherapy for neuroendocrine tumours, we postulated that a similar approach could offer a valid alternative in the diagnosis and therapy of GIST. Using in vitro receptor autoradiography to measure peptide receptors, we found that 16/19 GIST expressed bombesin subtype 2 receptors, 16/19 expressed vasoactive intestinal peptide subtype 2 receptors (VPAC(2)) and 12/19 expressed cholecystokinin subtype 2 receptors, in most cases in extremely high densities. All GIST metastases were shown to express two or more of these peptide receptors in very high density. Receptors were also expressed in non-responders to Glivec or after chemo-embolisation. Conversely, somatostatin subtype 2, cholecystokinin subtype 1, bombesin subtype 1 and 3, and neuropeptide Y subtype Y(1) and Y(2) receptors were not or only rarely expressed. These data represent a strong molecular basis for the use of radiolabelled bombesin, vasoactive intestinal peptide and/or cholecystokinin analogues as targeting agents to localise GIST tumours in patients by in vivo scintigraphy and/or to perform targeted radiotherapy to destroy GIST primaries, metastases and recurrences, including those resistant to Glivec