Knowledge of the anatomy of the internal mammary vessels is important to avoid hemorrhagic complications when an anterior parasternal approach is used for percutaneous transthoracic procedures such as biopsy and empyema drainage. We examined 100 consecutive CT scans of the thorax to assess both the number of internal mammary vessels and their relation to the sternum. The mean distance from the sternum to the most medial vessel, the internal mammary vein, was 1.03 +/- 0.25 cm on the right side and 0.98 +/- 0.23 cm on the left side. The mean distance from the sternum to the most lateral vessel, the internal mammary artery, was 1.57 +/- 0.30 cm on the right and 1.47 +/- 0.30 cm on the left. Three internal mammary vessels were present in 20% of cases on the right side and in 18% on the left side. In nine patients, the internal mammary artery was greater than 2.0 cm from the lateral border of the sternum. We recommend an approach that is greater than 2.5 cm from the sternal border when performing parasternal percutaneous transthoracic procedures in order to avoid hemorrhagic complications from injury to the internal mammary vessels. A "safe" window does exist medially between the sternal border and the internal mammary vein but should be used only in procedures performed under CT guidance.