Somatostatin receptor-expressing tumours are potential targets for therapy with radiolabelled somatostatin analogues. We have synthesized a number of such analogues in the past and identified [DOTA-dPhe1, Tyr3]octreotide (SMT 487) as the most promising candidate molecule because of its advantageous properties in cellular and in vivo tumour models. In the current paper we describe the radiotherapeutic effect of yttrium-90 labelled SMT 487 in Lewis rats bearing the somatostatin receptor-positive rat pancreatic tumour CA 20948. SMT 487 binds with nanomolar affinity to both the human and the rat somatostatin receptor subtype 2 (sst2) (human sst2 IC50=0.9 nM, rat sst2 IC50=0.5 nM). In vivo, 90Y-SMT 487 distributed rapidly to the sst2 expressing CA 20948 rat pancreatic tumour, with a tumour-to-blood ratio of 49.15 at 24 h post injection. A single intravenous administration of 10 mCi/kg 90Y-SMT 487 resulted in a complete remission of the tumours in five out of seven CA 20948 tumour-bearing Lewis rats. No regrowth of the tumours occurred 8 months post injection. Control animals that were treated with 30 microg/kg of unlabelled SMT 487 had to be sacrificed 10 days post injection due to excessive growth or necrotic areas on the tumour surface. Upon re-inoculation of tumour cells into those rats that had shown complete remission, the tumours disappeared after 3-4 weeks of moderate growth without any further treatment. The present study shows for the first time the curative potential of 90Y-SMT 487-based radiotherapy for somatostatin receptor-expressing tumours. Clinical phase I studies with yttrium-labelled SMT 487 have started in September 1997.