We have performed 100 scintigraphic investigations using [111In-diethylenetriaminepentaacetic acid-D-Phe1]octreotide (111In-octreotide) single photon emission tomography (SPECT) in patients with carcinoid tumors. One or several lesions could be detected in 77 cases, and true negative results were obtained in 11 cases. There were false-negative results in 12 cases compared with results from conventional radiological methods. The ratio between the SPECT signals from the area with the highest uptake and normal lung was used as a tumor:background ratio. An attenuation correction was made in all investigations. We found that lesions in untreated patients had lower tumor:background ratios compared with those in patients treated with somatostatin analogues (medians, 10 and 40, respectively; P < 0.001) or IFN (median, 23; P = 0.03). In untreated patients, there was a correlation between the tumor:background ratio and the levels of urinary 5-hydroxyindoleacetic acid (U-5HIAA) and p-chromogranin A. The data obtained in the present investigation indicate that somatostatin receptor expression might be influenced by the treatment; i.e., a higher tumor:background ratio is found in patients treated with either somatostatin analogues or IFN. Furthermore, it was found that somatostatin receptor expression correlates with the levels of U-5HIAA and p-chromogranin A in untreated patients, and that 111In-octreotide SPECT scintigraphy is more likely positive in patients with elevated U-5HIAA values. This indicates that somatostatin receptor expression and elevated U-5HIAA are more likely present in patients with highly differentiated tumors and, thus, could be of prognostic value.