Several types of neuroendocrine tumor express high numbers of somatostatin receptors (sstr). We have compared the expression of sstr subtypes with the outcome of octreotide scintigraphy in patients with carcinoids and medullary thyroid carcinoma (MTC) in comparison with Hürthle cell tumors. The effect of sstr activation (octreotide treatment) on tumor markers was also studied in patients with disseminated carcinoid tumors. Six patients with carcinoid tumors (four midgut and two foregut), and three patients with thyroid tumors (one MTC, one Hürthle cell carcinoma, and one Hürthle cell adenoma) were studied. Octreotide scintigraphy visualized tumor sites in all nine patients. Macroscopic tumor was verified at these sites at subsequent surgical exploration. Using Northern blotting and subtype-specific riboprobes, sstr could be detected in all tumors examined. All five sstr subtypes were detected in most of the carcinoid tumors. All six carcinoids expressed sstr2. This was in contrast to the findings for the thyroid tumors analyzed, which also expressed several sstr subtypes but in some cases lacked expression of sstr2. This was also the case for normal thyroid tissue. Clinically, octreotide treatment of the patients with midgut carcinoid tumors resulted in palliation of hormonal symptoms accompanied by a significant reduction of urinary 5-HIAA levels (28-71%). These results indicate that carcinoid tumors frequently express all five sstr subtypes. The thyroid tumors also expressed multiple sstr but could lack expression of sstr2. Nevertheless, these tumors were visualized by octreotide scintigraphy, indicating that sstr2 expression is not a prerequisite for tumor imaging.