The objective of this study was to evaluate the agreement among the US Department of Veterans Affairs (VA) listing of "critical" drug-drug interactions (DDIs) and two standard drug-interaction compendia. A list of critical DDIs, as defined by the VA, was compared with two standard commercially available compendia (Micromedex DRUG-REAX and Drug Interactions: Analysis and Management (DIAM)) in order to determine the level of agreement among the systems. Of 982 DDIs classified as critical by the VA, only 136 DDIs (13.7%) were considered critical in all three systems. Our conclusion was that, overall, there is poor agreement among the compendia and the VA system.