This study was aimed at assessing the clinical usefulness of measuring the contrast enhancement (CE) of solitary pulmonary nodules (SPN) in distinguishing benign from malignant lesions. We used spiral CT to study prospectively 35 pulmonary lesions presenting as SPNs < 30 mm phi; we evaluated the CE of the nodules 120 minutes after the administration of 100 mL of nonionic contrast material (= 30 grams of iodine), at 2 mL/s. The final diagnosis of the 35 SPNs was made at surgery (27 cases); positive sputum cytology (2 cases), 12 months' follow-up (5 cases) or fine-needle aspiration biopsy and 6 months' follow-up (1 case). Thus, 25 of 35 SPNs proved malignant (11 adenocarcinomas, 5 squamous cell carcinoma, 2 large cell carcinomas, 2 carcinoids, 1 small cell carcinoma, 2 cases with positive sputum cytology, 2 metastases) and the extant 10 of 35 proved benign. Malignant nodules enhanced markedly more (mean value: 36.8 HU) more than benign lesions (mean value: 18.6 HU). CE exceeded 20 HU in 23/25 malignant nodules and did not in 2/25; it did not exceed 20 HU in 6/10 benign nodules and did in 4/10. With 20 HU as the threshold value for a positive test (malignancy), sensitivity was 92%, specificity 60% and accuracy 83%; positive and negative predictive values were 85% and 75%, respectively. In conclusion, CE evaluation is a sensitive, although not very specific, indicator of malignancy in SPNs.