OBJECTIVE: To demonstrate an asymmetry of the internal jugular veins, a finding which will have consequences for catheterization. DESIGN: Prospective study. SETTING: The medical ICU of a university-affiliated teaching hospital. PATIENTS: Eighty critically ill consecutive patients. INTERVENTION: Measurement of the cross-sectional area of the internal jugular veins. Search for an asymmetry, defined as an area at least twice that of the contralateral vein. RESULTS: An asymmetry was noted in 62.5% of the patients. The dominant vein was the right in only 68 % of these cases. In addition, 23% of the 160 jugular internal veins had an area of 0.4 cm2 or less. CONCLUSIONS: Using a simple technique, ultrasound identifies the dominant internal jugular vein, thus indicating the safer side before blind catheterization.