Objective To evaluate whether underlying anatomical variations in the position of the internal jugular vein may account for difficulty in obtaining central venous access in individual patients. Design Consecutive series. Setting Cardiac catheterization laboratory, coronary care unit, and ICU. Patients Two hundred patients (52 ±PT 7 yrs, 147 males) who were undergoing internal jugular vein cannulation for hemodynamic monitoring or endomyocardial biopsy. Intervention The internal jugular vein and carotid artery were visualized with two-dimensional ultrasound and their position was compared with their projected location from external landmarks. Results In 183 (92%) patients, the position of the internal jugular vein was lateral and anterior to the carotid artery and increased in diameter during a Valsalva maneuver. In five (2.5%) patients, the internal jugular vein was not visualized and was probably thrombosed, as the internal jugular vein was normal on the other side. In six (3%) patients, the internal jugular vein was unusually small and did not increase in diameter during the Valsalva maneuver. In two (1%) patients, the internal jugular vein was positioned >1 cm lateral to the carotid artery. Four (2%) patients had a medially positioned internal jugular vein overlying the carotid artery. In 5.5% of the patients, the position of the internal jugular vein was outside the path that had been predicted by the external landmarks. Conclusions These findings suggest that anatomical variation may partly account for the inability to cannulate the internal jugular vein in certain patients. In these cases, ultrasound examination quickly establishes the position of the internal jugular vein and may allow for easy and rapid access.