Enlarged mediastinal, retroperitoneal and pelvic lymph nodes are often difficult to differentiate from vascular structures. Contrast medium is therefore used to help to discriminate arteries and veins from lymph nodes. This study was undertaken to investigate the degree to which pathologic lymph nodes become enhanced after an intravenous bolus injection of contrast medium. Computed tomography was performed in 25 patients with enlargement of retroperitoneal lymph nodes due to primary lympho-proliferative disease or metastases. A dynamic sequence of a well delineated lymph node was obtained over a period of two minutes. Contrast enhancement was seen in all lymph nodes, but of varying degree. The enhancement was correlated to that observed in the inferior vena cava. Most examined lymph nodes showed slight or moderate enhancement, but in five instances strong enhancement, more than 75 per cent of that of the vena cava, was found. These nodes could possibly have been misinterpreted as blood vessels.