CT-guided Percutaneous Transthoracic Biopsies (PTB) performed in the Radiology Department of Garcia de Orta Hospital between 2002 and 2004 to evaluate undetermined pulmonary lesions were retrospectively analysed. 89 fine needle aspiration biopsies (FNAB) and 13 core needle biopsies (CNB) were performed on 92 patients (67 men, mean age: 64.4 years). 82 lesions (89%) were nodular lesions (mean diameter: 3.8+/-1.7 cm, 65 peripheral). We did not observe complications among patients who underwent CNB; minor complications and pneumothorax requiring drainage occurred in 11 FNAB. 72 FNAB were considered adequate for cytology diagnosis; 72% of them positive for malignancy. All CNB were adequate and conclusive. From the 7 CNB performed on patients with previous FNAB, 3 allowed a better histological characterization and in 3 cases of inadequate FNAB, CNB was conclusive. All malignant lesions were nodules: 20 adenocarcinoma, 13 non-small cell lung cancer (SCLC), 10 epidermoid tumours, 5 small-cell lung cancer, 2 carcinoids, 1 bronchiolo alveolar carcinoma, 1 malignant mesothelioma and 8 metastasis. Unspecific/inflammatory lesions (n=5) were the most frequent benign lesions. Malignant lesions were more prevalent in older patients (p=0.007) and were larger (p=0.006). Spiculated and lobulated contour (p=0.05) were more prevalent in malignant lesions while regular contour was more frequent among benign lesions (p=0.0001). Gender, smoking, location, pleural tag, homogenous attenuation, cavitation, calcification, necrosis and air bronchogram did not differ significantly between benign and malignant nodules. This study shows that CT-guided PTB is a safe and effective procedure in the evaluation of undetermined pulmonary lesions.