Predictive tests for treatment with somatostatin analogues have been asked for by clinicians. We have shown previously that somatostatin receptor (sstr) scintigraphy may be used to predict therapeutic outcomes for carcinoid patients receiving somatostatin analogues. However, almost 20% of patients with pathological tracer uptake fail to respond to such treatment. To increase further the reliability and prognostic value of sstr identification, we investigated the presence of mRNA for the subtypes sstr1 and sstr2 by in situ hybridization on tumor specimen from 25 carcinoid patients (22 midgut, 2 foregut, and 1 hindgut), all receiving somatostatin analogue treatment (12 lanreotide, 8 octreotide, and 5 octastatin) and compared this to the therapeutic response evaluated as inhibition of hormone secretion. Expression of sstr2 mRNA could be detected in 15 patients, all responding to somatostatin analogue treatment and showing pathological tracer uptake in tumor lesions at sstr scintigraphy. In the remaining 10 patients, no sstr2 mRNA could be detected, and none of the patients responded to somatostatin analogue treatment. Three of these 10 patients failed to accumulate tracer activity at sstr scintigraphy, whereas 7 had a pathological uptake of [(111)In-DTPA-D-Phe(1)]-octreotide. We conclude that in this group of carcinoid patients, there was complete agreement between the presence of mRNA for sstr2 detected by in situ hybridization and therapeutic outcome. In patients with pathological tracer accumulation without expression of somatostatin sstr2 mRNA, other sstr may be present that can bind the somatostatin analogue but not inhibit hormone secretion.