Spiral computed tomography (CT) allows very satisfactory temporal resolution for the detection and analysis of changes in contrast medium density, so that volumes of contrast material smaller than those recommended for conventional CT can be used. The present double-blind, parallel group study was designed to compare image quality and diagnostic efficacy obtained with two different iodine strengths (200 and 300 mgI/ml) and two different flow rates (2 and 3 ml/s) of a fixed volume (70 ml) of iomeprol in adult consenting patients requiring spiral CT of the mediastinum. Imaging was performed during suspended respiration and, a 15-s delay to scan. Spiral CT was initiated 1 cm above the aortic arch and continued inferiorly for 24 cm in all patients. Two independent readers blindly graded image quality, opacification of the superior vena cava, thoracic aorta and pulmonary arteries, and overall diagnostic quality of the CT examination. CT density measurements were performed over the level of the aortic arch, left and right pulmonary arteries, left atrium, and descending aorta. Opacification of the mediastinal vessels was better in the patients injected at 3 ml/s flow rate and was independent of the iodine strength used, except in the case of the thoracic aorta, which was better opacified by injecting iomeprol 300 mgI/ml at 3 ml/s. The highest diagnostic efficacy of the spiral CT examination was obtained with iomeprol 300 mgI/ml at 3 ml/s infusion rate, which seems to represent the administration scheme of choice.