In response to a congressional request, GAO reviewed Department of Defense (DOD) efforts to identify and clean up abandoned hazardous waste disposal sites on a Navy and an Air Force installation in Guam. GAO found that DOD: (1) initiated its Installation Restoration Program (IRP) to identify suspected problems with closed disposal sites and to control the migration of hazardous contamination from those sites; (2) completed the program’s first phase, the identification of bases with potentially hazardous sites, at both facilities; (3) completed preliminary work on the second-phase confirmation study for the Air Force base; and (4) awarded a contract for a confirmation study for the naval base in April 1986. The Environmental Protection Agency (EPA) and Guam: (1) believe that both installations need to include more site assessment work; (2) questioned the scope of the first-phase assessments; (3) noted 45 sites requiring reexamination, despite earlier DOD determinations that they required no further study; and (4) identified more sites that DOD should have assessed during the first phase of IRP. GAO found that: (1) the Navy agreed to perform additional testing of 7 of its sites, but it did not agree to monitor an additional 27 sites, as Guam and EPA requested; (2) the Air Force and Guam were working together during the IRP second phase to ensure reassessment of sites the Air Force did not consider during the first phase; (3) testing of the air base’s drinking water has been sporadic and incomplete, but monthly testing of samples at various locations in the system is now in place; and (4) discussions are continuing between EPA, Guam, and the Navy concerning site monitoring.