The Defense Department (DOD), the Coast Guard, and the Customs Service all interdict illegal drugs--primarily cocaine--from South America. DOD is the lead agency, but all three agencies play a role in monitoring and detecting shipments of illegal drugs. The Coast Guard is the lead agency for apprehending ships that are smuggling drugs, with Customs providing help as needed. The Coast Guard and Customs share responsibility for apprehending aircraft involved in drug-smuggling. GAO could not identify the funds obligated and the number of flight hours and ship days used for drug interdiction in the drug transit zone because the three agencies do not routinely track this information. The results tracked by the three agencies to demonstrate their effectiveness of their drug interdiction efforts in the transit zone varied according to whether they focused on drug seizures or results of detection and monitoring and whether they were specific to the transit zone. Agencies can use several controls to ensure the accuracy of their own cocaine seizure data, such as assigning unique identification numbers to each seizure and headquarters review of data from field units. Although two interagency data systems have been developed to ensure the accuracy of governmentwide cocaine seizure data when multiple agencies participate in a seizure, the two systems do not prevent agencies from counting cocaine seizures in their own databases and annual counts when more than one agency participates in the seizure.