The separation performance of a filter element used in a hydraulic system to protect critical components can be seriously degraded by permitting fluid to by-pass the filter element. Such by-pass leakage paths stem from many different causes. For example, an element that is improperly sealed within the housing will permit a portion of the inlet flow to by-pass the filter. Damaged elements are another source of possible by-pass leakage. There is one leakage path, however, which is seldom considered in a contamination control analysis - by-pass valve leakage. Such a valve is placed in a filter housing to protect the element from excessive pressure differential and is sometimes referred to as a filter relief valve.This paper develops an analytical expression which describes the influence of leakage flow on the separation performance of a filter. Actual data associated with the flow-pressure drop characteristics of both commercially available filter elements and by-pass valves are utilized to illustrate the leakage flow relative to such valves. Graphical illustrations are used to demonstrate the performance deterioration due to by-pass valve leakage.