Introduction: In recent years, positron emission tomography (PET) has been increasingly applied in the diagnosis of neoplastic lung diseases. In contrast to conventional imaging studies, PET-CT enables the visualisation of not only the morphology of the suspicious lesion, but also its metabolism. The aim of the present study was to investigate the role of PET-CT in the initial assessment of patients with indeterminate solitary pulmonary lesions. Material and Methods: The study was conducted on a group of 82 patients with indeterminate lung nodule diagnosed at the National Institute of Tuberculosis and Lung Diseases in the period from January 2008 to May 2011. CT and PET-CT were performed in all of the patients. Histological or cytological examination of the biopsy specimens obtained from bronchoscopy, mediastinoscopy and intraoperatively were the reference tests. Results: Malignancy was documented in 40 patients (48.8%). Histopathological analysis of all tumours revealed 12 cases of squamous cell carcinoma, 18 cases of adenocarcinoma and 1 case of carcinoid, whereas in 9 patients the diagnosis of “non-small cell cancer not otherwise specified” was made. All lesions except one were of solid character on chest CT. SUVmax values exceeding 2.5 were found in 38 cancer patients (true positives, TP). The mean value of SUVmax was 9.1 (1–26.8). Forty-two lesions were documented as benign (51.2%). SUVmax values equal to or less than 2.5 were found in 37 patients (true negatives, TN). The mean value of SUVmax in this group was 1.9 (0.5–8.6). The diagnostic value of PET-CT SUVmax exceeding 2.5 in the prediction of neoplastic origin of solitary pulmonary lesions was: sensitivity—95% (95% CI 84–99%), specificity—88% (95% CI 75–95%) and accuracy—91.5% (95% CI 83–96%). Positive predictive value (PPV) was 88.4% (95% CI 76–95%), and negative predictive value (NPV) was 94.8% (95% CI 83–99%). False negative results concerned two patients, with final diagnosis of carcinoid and adenocarcinoma; false positive results were obtained in 5 patients with various inflammatory lesions. Conclusions: In the present study, PET-CT appeared to have high sensitivity (95%), but lower specificity (88%) for predicting the malignant character of solitary pulmonary lesions. Overall diagnostic value of PET-CT SUVmax > 2.5 was high—PPV was 88.4%, NPV was 94.8%. In the authors’ opinion, the PET-CT value may increase when clinical data as well as other radiological documentation (with retrospective assessment) are taken into consideration.