Forty-three consecutive patients with solitary pulmonary nodules (SPNs) on chest radiographs were studied scintigraphically after administration of the somatostatin analogue 99mTc-EDDA/HYNIC-TOC. The objective of the study was to assess the usefulness of the procedure for differentiation of SPNs as malignant or benign. The administered activity was 740-925 MBq, and a single-photon emission computed tomography imaging technique was employed. Verification of the nodule aetiology was based on histology or cytology and bacteriology. A stable tumour size on chest radiography for at least 3 years was accepted as an additional criterion of benignity. In 29 patients, nodules were found to be malignant. The diagnoses included ten adenocarcinomas, five squamous cell carcinomas, two large cell carcinomas, six non-small cell lung cancers without specification of the more detailed morphology, two small cell lung cancers, two typical carcinoids and two metastatic tumours (leiomyosarcoma and malignant melanoma). In 14 patients the following benign tumours were diagnosed: four tuberculomas, one other granuloma, three hamartomas, one non-specific inflammatory infiltrate, one abscess, one peripheral carcinoid with the morphological characteristics of a benign tumour, one ectopic lesion of thyroid tissue and two benign tumours of unspecified aetiology with a stable size over 3 and 5 years respectively. Positive scintigraphic results were obtained in 26 of the 29 patients (90%) with malignant SPNs; among these, 24 of the 25 (96%) cases of primary pulmonary carcinoma yielded positive results. The remaining two false negative cases were the metastatic tumours, liposarcoma and melanoma. Of the 14 benign lesions, ten (71%) did not accumulate the radiopharmaceutical. The remaining four benign tumours that were visible on scintigrams comprised one tuberculoma, one hamartoma, one abscess and one case in which the diagnosis could not be established (the tumour had a stable size over 3 years). In conclusion, scintigraphy with 99mTc-EDDA/HYNIC-TOC appears to be an effective procedure for differentiation between malignant and benign SPNs. A fully credible assessment of the clinical efficacy of this procedure requires further study in a larger number of patients.