Positron emission tomography (PET) using fluoride-18-marked fluoride deoxyglucose (FDG) represents a metabolically based imaging technique capable of providing information on the potential malignancy of peripheral pulmonary focal lesions. In the present prospective study, we investigated the effectiveness of FDG-PET in determining the dignities of 67 such lesions in 35 patients. Findings of FDG-PET were compared with those of computed tomography (CT), as well as with surgical and histological reports, and the value of FDG-PET as a diagnostic method evaluated. FDG-PET correctly identified 38 lesions as positive for malignancy, 18 correctly as negative, 7 incorrectly as negative, and 4 incorrectly as positive. Based on lesions, this yields a sensitivity of 84.4% and a specificity of 81.8%. All malignant focal lesions with a diameter of over 1.2 cm were correctly identified (sensitivity: 100%). In cases of intense FDG uptake, differentiation between a primary lesion, a metastasis, and an acute inflammation is often not possible.