A correct differential diagnosis between benign and malignant lesions is mandatory in patients with solitary pulmonary nodule (SPN). The aim of the present study was to investigate whether 99mTc-tetrofosmin SPECT may play a role in SPN evaluation. A consecutive series of 111 patients with an uncalcified </=3 cm (range: 0.8-3 cm) SPN, without definite benign findings and indeterminate at CT, were studied. Within 1 week of CT scan, following 740 MBq of 99mTc-tetrofosmin i.v. injection, all patients underwent chest SPECT using a rectangular dual head gamma camera with HR collimators. The images were analysed both qualitatively and semiquantitatively by calculating tumor/normal tissue ratio (T/N). All nodules were referred to a definitive diagnosis after scintigraphy: 84/111 nodules resulted malignant (primary lung carcinomas in 59 cases and metastases in 25), whereas 27/111 were benign. SPECT was true positive in 77/84 malignant nodules (overall sensitivity: 91.7%), detecting 55/59 carcinomas (93.2%) and 22/25 metastases (88%), whereas it was false negative in 4 carcinomas (3 adenocarcinomas and 1 squamous cell carcinoma, the latter with necrotic areas; range size: 1.5-2.4 cm) and in 3 metastases (range size: 1.0-1.2 cm). SPECT was true negative in 24/27 benign lesions (specificity: 88.9%) and false positive in 2 hamartomas and in 1 aspecific inflammation (range size: 0.8-2 cm), each with a T/N value </=1.4. Accuracy, positive predictive value and negative predictive value were 91, 96.2 and 77.4%, respectively. Mean T/N value was significantly higher in malignant than in benign nodules (2.1+/-0.6 vs. 1.3+/-0.1, P<0.05), whereas no significant differences were observed between primary lung carcinomas and metastases (2.1+/-0.6 vs. 1.9+/-0.6) or in the different histologic types of carcinomas. 99mTc-tetrofosmin SPECT proved a highly sensitive imaging method in both primary and secondary malignant </=3 cm SPNs detection, with a high accuracy value in discriminating malignant from benign lesions, also by adding semiquantitative analysis. A larger clinical application of this non-invasive, simple and widely available procedure is thus suggested in SPN management, especially when FDG-PET is not available.