PURPOSE: To determine whether the maximal enhancement time in dynamic CT is different between benign and malignant solitary pulmonary nodules (SPN)s, and to evaluate the value of densitometry on dynamic CT in predicting the malignancy of SPN. MATERIALS AND METHODS: Fifty-six patients with SPN of less than 4cm in diameter as seen on chest radiograph and SPN without benign pattern of calcification or fat, as seen on pre-enhancement spiral CT scans were included in this study. SPN with small cavitation sufficient to measure CT density, were also included.Thirty-four SPNs were diagnosed pathologically or radiologically as 20 malignant nodules and 14 benign nodules.Dynamic CT was performed by two techniques after injection of 50ml of nonionic contrast media at the rate of 2ml/sec. In 28 patients, incremental dynamic CT was performed before and of 15 seconds, 1 minute, 2 minutes, 3 minutes, and 4 minutes after injection of contrast media during shallow respiration. In 28 patients, double spiral CT was performed 2 minutes and 3 minutes after injection of contrast media during single breath hold. CT readings were taken at the central portion of SPNs, with a circular region of interest. The degree and time of maximal enhancement were recorded. RESULTS: In dynamic CT the maximal enhancement time of SPNs was not significantly different between malignant (2.73±1.27 minute) and benign nodules (2.56±1.24 minute). The enhancement of malignant nodules was significantly greater (21.42±12.17 HU) than of benign nodules (5.15±5.25 HU) (p<.0001). CONCLUSION: In dynamic CT of SPNs, there is no difference in maximal enhancement time between benign and malignant nodules ; enhancement of the latter is significantly greater than that of the former. Maximal enhancement greater than 15 HU can be a good predictor of malignancy of SPNs.