In a prospective study we examined 38 patients with primary bronchogenic carcinoma to validate the use of indium-111 pentetreotide (IPT) as a diagnostic tool. Of these 38 patients, 25 had small cell lung cancer (SCLC) and 13, non-small cell lung cancer (NSCLC). The aim of the study was to investigate whether (a) the disease can be reliably detected, (b) IPT allows differentiation between SCLC and NSCLC and (c) IPT provides further information on metastatic disease. After giving their informed consent the patients were injected and imaged 4 and 24 h later using a planar whole-body technique. In addition single-photon emission tomography of the thorax and, if necessary, other areas of the body was performed at 24 h. In the 25 patients with SCLC 22 sites of primary tumour were correctly identified (true-positive, TP); one was false-negative (FN) and two were true-negative (TN), the patients being in full remission. Metastases were correctly identified in ten instances (lung, bone and brain), while the findings were FN in five cases. An additional six FN findings resulted in the area of the upper abdomen due to the physiological uptake in the liver, spleen and kidneys. In the 13 patients with NSCLC, ten findings were TP and 3 FN with respect to the primary tumour. Two FNs were squamous cell carcinoma, and one, adenocarcinoma. Metastases were TP in nine cases and FN in one.(ABSTRACT TRUNCATED AT 250 WORDS)