OBJECTIVE Octreotide, a potent long-acting synthetic somatostatin analogue, has been reported to have a beneficial effect in thyroid eye disease (TED), but the precise mechanism of action remains unexplained. 111In-DTPA-D-Phe1-octreotide (Octreoscan-111) has been used to localize a number of endocrine tumours and visualize somatostatin receptors in the retrobulbar tissue of patients with TED. Furthermore, this technique can predict the inhibitory effect of octreotide on hormone secretion by endocrine tumours, as there is a close relation between the clinically observed inhibition and visualization of the tumour using Octreoscan-111. The aims of the present study were to confirm the beneficial effect of octreotide in patients with TED, to investigate the presence of somatostatin receptors in the orbital area and also, if possible, to ascertain whether this technique could select those patients with TED who might benefit from treatment with octreotide. METHODS A prospective study. METHODS An endocrine clinic of a national hospital. METHODS Twenty treated thyrotoxic patients with TED, 5 treated thyrotoxic patients without TED and 5 normal individuals were studied. In 12 patients with TED, 5 without TED and 5 normal individuals, Octreoscan-111 scintigraphy of the orbits was performed. The remaining 8 patients with ophthalmopathy served as controls. In patients with TED who were investigated with Octreoscan-111, 300 micrograms octreotide daily was given for 12 weeks. RESULTS Six patients in both eyes and one patient in one eye showed an improvement in ocular manifestations as assessed by clinical criteria and changes in the NOSPECS score, while the rest showed no improvement. The patients who showed an improvement had a high number of somatostatin receptors and positive orbital scans, while with one exception the patients who did not respond had a low number of receptors and negative orbital scans (P < 0.02). None of the 5 patients without TED nor the normal individuals had a positive orbital scan. Seven out of 8 control patients with TED showed no change in the disease during the trial, while 1 deteriorated. CONCLUSIONS We conclude that octreotide has a beneficial effect in thyroid eye disease and that Octreoscan-111 could predict those patients with thyroid eye disease who might benefit from this treatment.