The multiple aspects of radioligand-receptor interactions do not only show a major impact of certain cell surface-bound receptors in the pathophysiology of human disease: the concept of radioligand-receptor interactions has also been extended to the clinic. In particular, naturally occurring peptides, when radiolabelled, are clinically useful for the imaging diagnosis of human disease and have future implications for the treatment of tumour expressing certain target receptors using radiolabelled peptide tracers. The finding that receptors for VIP (vasoactive intestinal peptide) and SST (somatostatin) are overexpressed on tumour cells presents a breakthrough into this direction. Recent data indicate that [123I]-VIP receptor scintigraphy is clinically useful for the in vivo localization of small primary adenocarcinomas, liver metastases and certain endocrine tumours of the gastrointestinal tract. After the successful clinical introduction of the SST analogues [123I]-Tyr3-octreotide and [111In]-DTPA-D-Phe1-octreotide for localization diagnosis of neuroendocrine tumours in 1989, P829, labelled with the more cost-effective radionuclide 99mTc, nowadays promises to be a potential novel diagnostic imaging agent for tumours expressing SST/VIP receptors. Furthermore, the novel SST analogue [90Y]-MAURITIUS is entering the clinic for treatment of VIP/SST receptor-expressing tumours.