In this study Tl-201 planar scintigraphy and single photon emission computed tomography (SPECT) were performed in 92 patients with solitary lung lesions in order to distinguish malignant versus benign and metastatic masses and to evaluate the radiotherapy response of 15 primary lung carcinomas. Semiquantitative and quantitative analyses were carried out and the results were compared with histopathological diagnosis. The overall sensitivity, specificity and accuracy of semiquantitative analysis were 56, 71 and 58%, and quantitative evaluations were 88.8, 100 and 92%, respectively. Retention indexes (RI) derived from the early and delayed images were 2.9 +/- 1.3 for primary lung carcinomas, -2.78 +/- 0.9 for benign conditions and -2.3 +/- 1.5 for metastatic pulmonary lesions. Tl-201 scintigraphy seems to be a sensitive modality for differentiating malignancies from benign conditions since the difference between those two pathologies is significant. However, this procedure has a limited value in distinguishing metastatic pulmonary lesions from benign pathologies, and in tissue characterization of primary lung carcinomas. On the other hand, a good correlation was observed between Tl-201 lung scintigraphy and clinical diagnostic examination during postradiotherapy follow-up of inoperable pulmonary neoplasia.