No detailed descriptions exist of the collateral intercostal artery which can provide an accurate anatomical basis for ensuring a low rate of vascular complications during thoracocentesis and thoracoscopy. Consequently the present study was undertaken to provide information on the origin, size and topographic relationships of the collateral intercostal artery. Ninety cadaveric adult intercostal spaces were dissected using standard procedures. The collateral intercostal arteries were exposed throughout their full length and measured at the points within the intercostal spaces considered to be the most important for clinical purposes. The posterior intercostal arteries and their collateral branches were observed to decrease in size from posterior to anterior; however, no significant differences were present in their size in the first four intercostal spaces. Based on these findings the usual thoracocentesis or thoracoscopy technique, in which it is recommended that puncture is done as close as possible to the superior margin of the inferior rib, may lead to collateral intercostal artery laceration and considerable bleeding. A subtle, but important, modification to this technique is suggested aimed at achieving safer access to the intercostal space. In the modified technique, the puncture should be made in the middle of the intercostal space, thereby reducing the possibility of laceration of the collateral intercostal artery.