Purpose : To evaluate the effect of reduced volume of contrast media on vascular opacification and imagequality in spiral CT of the chest. Meterials and Methods : Sixty patients referred for chest CT were examined withspiral CT with 60ml(n=30) or 90ml(n=30) of 30% ionic contrast media(Rayvist 300 , Schering, Germany) alternately.Injection rate of each group was as follows : 2.0 mlsec for 20 seconds followed by 1.0 ml/sec for 20 seconds in60ml group and 2.0mlsec for 45 seconds in 90ml group. Twenty-five seconds scanning delay was employed. For theobjective comparison of vascular opacification, CT numbers were measured at superior vena cava, ascending anddescending aorta, right and left pulmonary artery, left atrium, and inferior vena cava. For the subjectivecomparison three radiologists scored the grade of vascular opacification and image quality blindly andindependently. All data were analyzed statistically. Results : The mean values of measured CT numbers in 90mlgroup were higher than those in 60ml group(p〈.05) at the same level. The overall mean score of vascularopacification in 90ml group was 2.86, and 2.31 in 60ml group(p〈.0001). In the overall mean score of imagequality, there was no statistically significant difference between 90ml group(2.46) and 60ml group(2.40).Conclusion : Althought there is some degradation of vascular opacification in 60ml group, overall image quality isnot degraded. Therefore, 60 ml of contrast media can be used in spiral CT of the chest without degradation ofoverall image quality except in spiral CT angiography.