Nine different brands representing the four main types of modern portable drainage systems (expanding suction balls, suction bellows, spring-loaded systems and high vacuum systems) were examined with respect to the relationship between the degree of filling of the vacuum source and the resulting vacuum. The results are presented first graphically and second numerically with five figures according to a system suggested by the authors. It is further concluded that the vacuum-volume curves, except for the high vacuum systems, presented by the manufacturers are not always in full accordance with the findings in this study. When a surgeon chooses to use a drain in a certain situation it is of great importance, on the one hand, that he has a knowledge of the degree of vacuum required for that particular situation and, on the other hand, which drains will fulfill these requirements. These problems are discussed in detail. This article provides baseline data for the different kinds of portable drains currently available and thus gives a background for rational decision making.