Although visible collateral vessels on computed tomography (CT) has been considered as an important finding in superior vena cava (SVC) syndrome, there is no systematical analysis concerning correlation between the CT evidence of collateral vessels and clinical evidence of SVC syndrome. The purpose of this study is to evaluate how accurately we predict the clinical presence of SVC syndrome by the collateral vessels in patients with apparent SVC obstruction on CT. Forty-seven patients having a CT evidence of obstruction or compression of SVC and / or its major tributaries were included in this study. Lung cancer was the most common underlying disease (n=40). The enhanced CT scans were obtained through either arm vein using a combined bolus and drip-infusion technique. Analying the CT scans, we particularly paid attention to the site and pattern of venous compromise, presence of collateral vessels, and if present, their location, without knowing whether symptoms and signs were present or not, and then compared them with clinical data by a thorough review of charts. To verify the frequency of visible collateral vessel in normal subjects, we also evaluated the CT scans of 50 patients without mediastinal disease and clinical SVC syndrome as a control group On CT, collateral vessels were found in 24 patients, among whom three patients had a single collateral and 21 patients had two or more collateral channels. There were two false positive cases, in which clinically overt SVC syndrome appeared 10days and three months after CT examination respectively, and one false negative case. The presence of collateral vessels on CT. respectively, and one false negative case. The presence of collateral vessels on CT, regardless of the nmber and location of collateral vessels and the pattern of venous obstruction, was a good clue for predicting the presence of clinical SVC syndrome with the sensitivity and the specificity of 95.7% and 91.7% respectively. In control group, collateral vessels were seen in three patients (6%) We conclude that the presence of collateral vessels on CT is a highly sensitive and specific sign of clinical SVC syndrome.