A: U.S. Government studies have concluded that the single-issue (one question) polygraph exam, conducted properly by a qualified examiner, is 87 to 95 percent accurate.  It is the most accurate tool available today for determining truth or deception.  Accuracy of the multi-question exam drops to around 80 percent due to a number of psychological factors.  These statistics do not include "inconclusive" results in which no opinion can be made from the polygraph charts, which happens about 20% of the time. 
For more information click here: Validity and Realiability of Polygraph Testing .